[Intel-gfx] cmdparser perf improvement

[Chris Wilson]

>From the moment the cmdparser was enabled (4.0) we got regression reports
about the performance regression, e.g. most notable on Baytrail

	http://www.spinics.net/lists/dri-devel/msg80933.html
	msg->id:1428627643.3417.22.camel at collabora.com

Whilst this doesn't make the cmdparser free, it does significantly
reduce the overhead. (The cached vmappings and better hash were tested
at the time and demonstrated to reduce the impact on the user's workload
to the point where the new kernel was an improvement over the last known
good). This builds upon the regression fixes to stop the cmdparser
falling over in the first place.
-Chris