i965: Calculate thread_count in brw_alloc_stage_scratch

Previously, thread_count was sent in from the stage after some stage
specific calculations. Those stage specific calculations were moved
into brw_alloc_stage_scratch, which will allow the shader cache to
also use the same calculations.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit f9d5a7ad)