video_core/gpu_thread: Implement a ShutDown method.

This was implicitly done by `is_powered_on = false`, however the explicit method allows us to block until the GPU is actually gone.

This should fix a race condition while removing the other subsystems while the GPU is still active.

src/core/core.cpp

@@ -296,7 +296,7 @@ struct System::Impl {
         exit_lock = false;
 
         if (gpu_core) {
-            gpu_core->WaitIdle();
+            gpu_core->ShutDown();
         }
 
         services.reset();