i965: Keep track of the per-thread scratch allocation in brw_stage_state.

i965: Keep track of the per-thread scratch allocation in brw_stage_state.

This will be used to find out what per-thread slot size a previously
allocated scratch BO was used with in order to fix a hardware race
condition without introducing additional stalls or memory allocations.
Instead of calling brw_get_scratch_bo() manually from the various
codegen functions, call a new helper function that keeps track of the
per-thread scratch size and conditionally allocates a larger scratch
BO.

v2: Handle BO allocation manually instead of relying on
    brw_get_scratch_bo (Ken).

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>