kms/update: Allow page flip callback listeners on any thread

This hooks into the MetaThread GMainContext callback machinery just
added; and allows receiving page flip callbacks on any thread.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2777>

src/backends/native/meta-kms-impl-device-atomic.c

@@ -755,6 +755,7 @@ process_page_flip_listener (MetaKmsImplDevice  *impl_device,
   meta_kms_page_flip_data_add_listener (page_flip_data,
                                         listener->vtable,
                                         listener->flags,
+                                        listener->main_context,
                                         listener_user_data,
                                         listener_destroy_notify);