backend/drm: add wlr_drm_connector_state.conn

Allows one to get back the connector from its state. Avoids having
to pass both structs around as function arguments.

include/backend/drm/drm.h

@@ -93,6 +93,7 @@ struct wlr_drm_mode {
 };
 
 struct wlr_drm_connector_state {
+	struct wlr_drm_connector *conn;
 	const struct wlr_output_state *base;
 	bool modeset;
 	bool active;