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/core/hle/service/nvflinger/nvflinger.h

@@ -102,6 +102,8 @@ private:
 
     u32 swap_interval = 1;
 
+    bool first_buffer_enqueued{};
+
     /// Event that handles screen composition.
     Core::Timing::EventType* composition_event;