backend/drm: make drmModePlane arg in add_plane const

Make it clearer it'll be free'd after add_plane returns.

backend/drm/drm.c

@@ -89,7 +89,7 @@ bool check_drm_features(struct wlr_drm_backend *drm) {
 }
 
 static bool add_plane(struct wlr_drm_backend *drm,
-		struct wlr_drm_crtc *crtc, drmModePlane *drm_plane,
+		struct wlr_drm_crtc *crtc, const drmModePlane *drm_plane,
 		uint32_t type, union wlr_drm_plane_props *props) {
 	assert(!(type == DRM_PLANE_TYPE_PRIMARY && crtc->primary));
 	assert(!(type == DRM_PLANE_TYPE_CURSOR && crtc->cursor));