kms/update: Change lock() to seal()

At first it was called seal(), but then updates could be amended after
being posted, given a flag. That flag has been removed, so we can go
back to sealing, since it's once again acts more as a seal.

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

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

@@ -309,6 +309,8 @@ meta_kms_device_process_update_sync (MetaKmsDevice     *device,
   MetaKms *kms = META_KMS (meta_kms_device_get_kms (device));
   PostUpdateData data;
 
+  meta_kms_update_seal (update);
+
   data = (PostUpdateData) {
     .update = update,
     .flags = flags,