kms: Skip displays with 'non-desktop' property set

Detect displays marked as 'non-desktop' by the kernel and skip them when
creating the outputs. Mutter is not able to render images that are shown
properly on those devices anyway.

This avoids lighting up attached VR HMDs and showing the GDM login
screen between the eyes in a VR HMD instead of on the monitor.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1393

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

@@ -506,7 +506,7 @@ init_outputs (MetaGpuKms *gpu_kms)
       GError *error = NULL;
 
       connector_state = meta_kms_connector_get_current_state (kms_connector);
-      if (!connector_state)
+      if (!connector_state || connector_state->non_desktop)
         continue;
 
       old_output =

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

@@ -206,6 +206,9 @@ state_set_properties (MetaKmsConnectorState *state,
       else if ((prop->flags & DRM_MODE_PROP_ENUM) &&
                strcmp (prop->name, "panel orientation") == 0)
         set_panel_orientation (state, prop, drm_connector->prop_values[i]);
+      if ((prop->flags & DRM_MODE_PROP_RANGE) &&
+          strcmp (prop->name, "non-desktop") == 0)
+        state->non_desktop = drm_connector->prop_values[i];
 
       drmModeFreeProperty (prop);
     }

src/backends/native/meta-kms-connector.h

@@ -49,6 +49,7 @@ typedef struct _MetaKmsConnectorState
   GBytes *edid_data;
 
   gboolean has_scaling;
+  gboolean non_desktop;
 
   CoglSubpixelOrder subpixel_order;