backend/drm: add wlr_drm_connector_state.connector

Will be useful for multi-connector commits, to avoid having to
pass the list of connectors separately from their state.

include/backend/drm/drm.h

@@ -123,6 +123,7 @@ struct wlr_drm_mode {
 };
 
 struct wlr_drm_connector_state {
+	struct wlr_drm_connector *connector;
 	const struct wlr_output_state *base;
 	bool modeset;
 	bool nonblock;