video_core/gpu: Create threads separately from initialization

Like with CPU emulation, we generally don't want to fire off the threads
immediately after the relevant classes are initialized, we want to do
this after all necessary data is done loading first.

This splits the thread creation into its own interface member function
to allow controlling when these threads in particular get created.

src/video_core/gpu_thread.h

@@ -138,10 +138,12 @@ struct SynchState final {
 /// Class used to manage the GPU thread
 class ThreadManager final {
 public:
-    explicit ThreadManager(Core::System& system, VideoCore::RendererBase& renderer,
-                           Tegra::DmaPusher& dma_pusher);
+    explicit ThreadManager(Core::System& system);
     ~ThreadManager();
 
+    /// Creates and starts the GPU thread.
+    void StartThread(VideoCore::RendererBase& renderer, Tegra::DmaPusher& dma_pusher);
+
     /// Push GPU command entries to be processed
     void SubmitList(Tegra::CommandList&& entries);