i965: Account for poor address calculations in Haswell CS scratch size.

i965: Account for poor address calculations in Haswell CS scratch size.

Curro figured this out by investigating the simulator.  Apparently
there's also a workaround in the Windows driver.  I'm not sure it's
actually documented anywhere.

We were underallocating the scratch buffer by a factor of 128/70.

v2: Rename threads_per_subslice to scratch_ids_per_subslice
    (suggested by Jordan Justen).

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>