anv/pipeline: Fix MEDIA_VFE_STATE::PerThreadScratchSpace on gen7

We were always programming it with the Broadwell convention which is too
large by a factor of two on Haswell and just plain wrong on IVB and BYT.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 7eaaff18)