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