backend/drm: damage outputs when switching CRTCs

backend/drm/drm.c

@@ -835,6 +835,8 @@ void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs) {
 		}
 
 		drm_connector_start_renderer(conn);
+
+		wlr_output_damage_whole(&conn->output);
 	}
 }