Fifo: Run/sync with the GPU on command processor register access

Source/Core/VideoCommon/Fifo.h

@@ -33,6 +33,10 @@ enum class SyncGPUReason
 // In deterministic GPU thread mode this waits for the GPU to be done with pending work.
 void SyncGPU(SyncGPUReason reason, bool may_move_read_ptr = true);
 
+// In single core mode, this runs the GPU for a single slice.
+// In dual core mode, this synchronizes with the GPU thread.
+void SyncGPUForRegisterAccess();
+
 void PushFifoAuxBuffer(const void* ptr, size_t size);
 void* PopFifoAuxBuffer(size_t size);