i965: Fix max_wm_threads for CHV

i965: Fix max_wm_threads for CHV

Change max_wm_threads to match the spec on CHV. The max number of
threads in 3DSTATE_PS is always programmed to 64 and the hardware
internally scales that depending on the GT SKU. So this doesn't
change the max number of threads actually used, but it does affect
the scratch space calculation.

On CHV the old value was too small, so the amount of scratch space
allocated wasn't sufficient to satisfy the actual max number of
threads used.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>