backend/drm: add wlr_drm_connector.backend

This allows the DRM code to have direct access to the wlr_drm_backend
without having to go through an upcast via get_drm_backend_from_backend.

include/backend/drm/drm.h

@@ -111,6 +111,7 @@ struct wlr_drm_mode {
 struct wlr_drm_connector {
 	struct wlr_output output;
 
+	struct wlr_drm_backend *backend;
 	enum wlr_drm_connector_state state;
 	struct wlr_output_mode *desired_mode;
 	bool desired_enabled;