drm-buffer: Don't always generate fb_id on construction

It might not be needed by the user of the buffer, so don't always
require it up front. Instead make sure that any user that needs it first
calls "meta_drm_buffer_ensure_fb_id()" to create the ID.

Only the plain gbm implementation creates the ID lazilly, the other
still does it on construction due to the objects used to create them
only existing during construction.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1854>

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

@@ -421,6 +421,9 @@ process_plane_assignment (MetaKmsImplDevice  *impl_device,
 
   buffer = plane_assignment->buffer;
 
+  if (buffer && !meta_drm_buffer_ensure_fb_id (buffer, error))
+    return FALSE;
+
   meta_topic (META_DEBUG_KMS,
               "[atomic] Assigning %s plane (%u, %s) to %u, "
               "%hdx%hd+%hd+%hd -> %dx%d+%d+%d",