This is an important optimization for avoiding read-after-write (RAW)
stalls in the HiZ buffer.  Certain workloads would run very slowly with
HiZ enabled, but run much faster with the "hiz=false" driconf option.
With this patch, they run at full speed even with HiZ.

Increases performance in OglVSInstancing by about 2.7x on Braswell.
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>