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_thread.h

@@ -132,8 +132,8 @@ public:
     /// Notify rasterizer that any caches of the specified region should be flushed and invalidated
     void FlushAndInvalidateRegion(VAddr addr, u64 size);
 
-    // Wait until the gpu thread is idle.
-    void WaitIdle() const;
+    // Stops the GPU execution and waits for the GPU to finish working
+    void ShutDown();
 
     void OnCommandListEnd();
 
@@ -141,6 +141,9 @@ private:
     /// Pushes a command to be executed by the GPU thread
     u64 PushCommand(CommandData&& command_data);
 
+    // Wait until the gpu thread is idle.
+    void WaitIdle() const;
+
     Core::System& system;
     const bool is_async;
     VideoCore::RasterizerInterface* rasterizer = nullptr;