Instead of having a pending_fb field on the struct wlr_drm_plane, move it to struct wlr_drm_connector_state. That way, there's no risk having a stale pending FB around: the state doesn't survive across tests and commits.
The cursor is a special case because it's disconnected from the atomic state: the wlr_backend_impl.set_cursor hook sets the cursor for the next commit. Move the field to wlr_drm_connector.cursor_pending_fb.