GPU_Async: Correct fences, display events and more.

This commit uses guest fences on vSync event instead of an articial fake 
fence we had.
It also corrects to keep signaling display events while loading the game 
as the OS is suppose to send buffers to vSync during that time.

src/video_core/gpu.h

@@ -177,6 +177,9 @@ public:
     /// Returns a reference to the GPU DMA pusher.
     Tegra::DmaPusher& DmaPusher();
 
+    /// Allows the CPU/NvFlinger to wait on the GPU before presenting a frame.
+    void WaitFence(u32 syncpoint_id, u32 value) const;
+
     void IncrementSyncPoint(u32 syncpoint_id);
 
     u32 GetSyncpointValue(u32 syncpoint_id) const;