ProcessorInterface: sync GPU just before PI_FIFO_RESET

GXAbortFrame() is problematic for Dolphin because it first writes
PI_FIFO_RESET (for which we discard our internal fifo), then disables CP
reads (for which we execute pending commands in the GP fifo in emulated
memory). I don't know whether there is a race condition on hardware, but
there is one for us. Avoid this by also doing a GPU sync here.

Source/Core/Core/HW/ProcessorInterface.cpp

@@ -98,6 +98,10 @@ void ProcessorInterfaceManager::RegisterMMIO(MMIO::Mapping* mmio, u32 base)
                    {
                      system.GetGPFifo().ResetGatherPipe();
 
+                     // Assume that all bytes that made it into the GPU fifo did in fact execute
+                     // before this MMIO write takes effect.
+                     system.GetFifo().SyncGPUForRegisterAccess();
+
                      // Call Fifo::ResetVideoBuffer() from the video thread. Since that function
                      // resets various pointers used by the video thread, we can't call it directly
                      // from the CPU thread, so queue a task to do it instead. In single-core mode,