kms: Prepare shutdown on 'prepare-shutdown' signal

Doing it in dispose means the backend is actively tearing down itself,
meaning various components might or might not be there, depending on how
the tearing down is implemented. Make things a bit more robust by doing
any work that might rely on the backend being there before shutdown is
done in response to the 'prepare-shutdown' signal being emitted by the
backend.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2853>

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

@@ -113,9 +113,6 @@ meta_backend_native_dispose (GObject *object)
 {
   MetaBackendNative *native = META_BACKEND_NATIVE (object);
 
-  if (native->kms)
-    meta_kms_prepare_shutdown (native->kms);
-
   G_OBJECT_CLASS (meta_backend_native_parent_class)->dispose (object);
 
   g_clear_pointer (&native->startup_render_devices, g_hash_table_unref);