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/video_core/gpu.cpp

@@ -517,8 +517,8 @@ void GPU::TriggerCpuInterrupt(const u32 syncpoint_id, const u32 value) const {
     interrupt_manager.GPUInterruptSyncpt(syncpoint_id, value);
 }
 
-void GPU::WaitIdle() const {
-    gpu_thread.WaitIdle();
+void GPU::ShutDown() {
+    gpu_thread.ShutDown();
 }
 
 void GPU::OnCommandListEnd() {