kms/update: Support merging updates with mode sets

Will be helpful to allow constructing each initial mode set update per
CRTC, merging them later on-demand.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2854>

src/backends/native/meta-kms-update.c

@@ -671,6 +671,56 @@ meta_kms_custom_page_flip_free (MetaKmsCustomPageFlip *custom_page_flip)
   g_free (custom_page_flip);
 }
 
+static GList *
+find_mode_set_link_for (MetaKmsUpdate *update,
+                        MetaKmsCrtc   *crtc)
+{
+  GList *l;
+
+  for (l = update->mode_sets; l; l = l->next)
+    {
+      MetaKmsModeSet *mode_set = l->data;
+
+      if (mode_set->crtc == crtc)
+        return l;
+    }
+
+  return NULL;
+}
+
+static void
+merge_mode_sets (MetaKmsUpdate *update,
+                 MetaKmsUpdate *other_update)
+{
+  while (other_update->mode_sets)
+    {
+      GList *l = other_update->mode_sets;
+      MetaKmsModeSet *other_mode_set = l->data;
+      MetaKmsCrtc *crtc = other_mode_set->crtc;
+      GList *el;
+
+      other_update->mode_sets =
+        g_list_remove_link (other_update->mode_sets, l);
+
+      el = find_mode_set_link_for (update, crtc);
+      if (el)
+        {
+          meta_kms_mode_set_free (el->data);
+          update->mode_sets =
+            g_list_insert_before_link (update->mode_sets, el, l);
+          update->mode_sets =
+            g_list_delete_link (update->mode_sets, el);
+        }
+      else
+        {
+          update->mode_sets =
+            g_list_insert_before_link (update->mode_sets,
+                                       update->mode_sets,
+                                       l);
+        }
+    }
+}
+
 static GList *
 find_plane_assignment_link_for (MetaKmsUpdate *update,
                                 MetaKmsPlane  *plane)
@@ -808,10 +858,10 @@ meta_kms_update_merge_from (MetaKmsUpdate *update,
                             MetaKmsUpdate *other_update)
 {
   g_return_if_fail (update->device == other_update->device);
-  g_return_if_fail (!update->mode_sets && !other_update->mode_sets);
   g_return_if_fail (!update->connector_updates &&
                     !other_update->connector_updates);
 
+  merge_mode_sets (update, other_update);
   merge_plane_assignments_from (update, other_update);
   merge_crtc_color_updates_from (update, other_update);
   merge_custom_page_flip_from (update, other_update);

src/tests/native-kms-updates.c

@@ -408,6 +408,8 @@ meta_test_kms_update_merge (void)
   MetaKmsUpdate *update2;
   g_autoptr (MetaGammaLut) lut = NULL;
   g_autoptr (MetaDrmBuffer) cursor_buffer2 = NULL;
+  GList *mode_sets;
+  MetaKmsModeSet *mode_set;
   GList *plane_assignments;
   MetaKmsPlaneAssignment *plane_assignment;
   GList *crtc_color_updates;
@@ -456,12 +458,17 @@ meta_test_kms_update_merge (void)
                                                 10, 11);
 
   /*
-   * Create an update2 with with cursor buffer 2
+   * Create an update2 with a mode set and a cursor buffer 2
    * on the cursor plane at at (32, 56), and a new CRTC gamma.
    */
 
   update2 = meta_kms_update_new (device);
 
+  meta_kms_update_mode_set (update2,
+                            crtc,
+                            g_list_append (NULL, connector),
+                            mode);
+
   lut = meta_gamma_lut_new (3,
                             (uint16_t[]) { 1, 2, 3 },
                             (uint16_t[]) { 4, 5, 6 },
@@ -488,6 +495,14 @@ meta_test_kms_update_merge (void)
   meta_kms_update_merge_from (update1, update2);
   meta_kms_update_free (update2);
 
+  mode_sets = meta_kms_update_get_mode_sets (update1);
+  g_assert_cmpuint (g_list_length (mode_sets), ==, 1);
+  mode_set = mode_sets->data;
+  g_assert (mode_set->crtc == crtc);
+  g_assert (mode_set->mode == mode);
+  g_assert_cmpuint (g_list_length (mode_set->connectors), ==, 1);
+  g_assert (mode_set->connectors->data == connector);
+
   plane_assignments = meta_kms_update_get_plane_assignments (update1);
   g_assert_cmpuint (g_list_length (plane_assignments), ==, 2);
   plane_assignment = meta_kms_update_get_primary_plane_assignment (update1,