i965: Hard code CS scratch_ids_per_subslice for Cherryview

Ken suggested that we might be underallocating scratch space on HD
400. Allocating scratch space as though there was actually 8 EUs
seems to help with a GPU hang seen on synmark CSDof.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104636
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105290


Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Eero Tamminen <eero.t.tamminen@intel.com>
(cherry picked from commit 06e3bd02)