backend/drm: destroy backend on udev remove event

Any use of the DRM FD after the remove event results in a "Permission
denied" error.

include/backend/drm/drm.h

@@ -88,6 +88,7 @@ struct wlr_drm_backend {
 	struct wl_listener session_active;
 	struct wl_listener parent_destroy;
 	struct wl_listener dev_change;
+	struct wl_listener dev_remove;
 
 	struct wl_list fbs; // wlr_drm_fb.link
 	struct wl_list outputs;