drm/i915: Improve HiZ throughput on Cherryview.

Found by reading the HIZ_CHICKEN documentation.

Improves performance in a HiZ microbenchmark by around 50%.
Improves performance in OglZBuffer by around 18%.

Thanks to Chris Wilson for helping me figure out where to put this.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>