VideoCommon: disable efb access + perf querys on cph thread

The usual way to handle this kind of request is to rise a flag which the gpu thread polls.
The gpu thread itself either generates the result or just write zeros if disabled.
After this, it rise another flag which says that this work is done.

So if disabled, we still have the cpu-gpu round trip time. This commit just returns 0 on the cpu thread
instead of playing ping pong...

Source/Core/VideoCommon/PerfQueryBase.h

@@ -29,6 +29,7 @@ public:
 	virtual ~PerfQueryBase() {}
 
 	// Checks if performance queries are enabled in the gameini configuration.
+	// NOTE: Called from CPU+GPU thread
 	bool ShouldEmulate() const;
 
 	// Begin querying the specified value for the following host GPU commands