i965: Allocate scratch space for the maximum number of compute threads.

i965: Allocate scratch space for the maximum number of compute threads.

We were allocating enough space for the number of threads per subslice,
when we should have been allocating space for the number of threads in
the entire GPU.

Even though we currently run with a reduced thread count (due to a bug),
we might still overflow the scratch buffer because the address
calculation is based on the FFTID, which can depend on exactly which
threads, EUs, and threads are executing.  We need to allocate enough
for every possible thread that could run.

Fixes rendering corruption in Synmark's Gl43CSDof on Gen8+.
Earlier platforms need additional bug fixes.

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>