UI: maintain frequency before vipc is connected (#21704)

Co-authored-by: Comma Device <device@comma.ai>

selfdrive/ui/ui.cc

@@ -224,6 +224,8 @@ static void update_vision(UIState *s) {
     } else if (!Hardware::PC()) {
       LOGE("visionIPC receive timeout");
     }
+  } else if (s->scene.started) {
+    util::sleep_for(1000. / UI_FREQ);
   }
 }