renderer/native: Don't drop disable-only mode set updates

If we are making an update that only disables CRTCs, we would not
actually post it, but just drop it then post nothing, as it wasn't ever
added to the mode set update hash table. This resulted in hotplugs where
we loose the all the connectors we had, where we want to disable all
CRTCs and enable nothing, to fail to disable said CRTCs.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3073>

src/backends/native/meta-renderer-native.c

@@ -287,6 +287,8 @@ ensure_mode_set_update (MetaRendererNative *renderer_native,
     return kms_update;
 
   kms_update = meta_kms_update_new (kms_device);
+  g_hash_table_insert (renderer_native->mode_set_updates,
+                       kms_device, kms_update);
 
   return kms_update;
 }