Fifo: rewrite sync on idle skipping hack

Now it's done without a busy loop

Source/Core/VideoCommon/MainBase.cpp

@@ -233,9 +233,9 @@ void VideoBackendHardware::Video_GatherPipeBursted()
 	CommandProcessor::GatherPipeBursted();
 }
 
-bool VideoBackendHardware::Video_IsPossibleWaitingSetDrawDone()
+void VideoBackendHardware::Video_Sync()
 {
-	return CommandProcessor::isPossibleWaitingSetDrawDone;
+	FlushGpu();
 }
 
 void VideoBackendHardware::RegisterCPMMIO(MMIO::Mapping* mmio, u32 base)