backend/drm: fix libliftoff_plane double-free

Destroying the liftoff_device invalidates all of the liftoff_planes.
Destroy the liftoff_planes before the liftoff_device to fix this.

backend/drm/libliftoff.c

@@ -109,6 +109,11 @@ static void finish(struct wlr_drm_backend *drm) {
 		liftoff_output_destroy(crtc->liftoff);
 	}
 
+	for (size_t i = 0; i < drm->num_planes; i++) {
+		struct wlr_drm_plane *plane = &drm->planes[i];
+		liftoff_plane_destroy(plane->liftoff);
+	}
+
 	liftoff_device_destroy(drm->liftoff);
 }