intel: Reduce cost of resetting last_grf_write.

intel: Reduce cost of resetting last_grf_write.

In zink-on-anv fs-mod-dvec3-dvec3.shader_test, we were memsetting 2MB of
last_grf_write 2400 times, multiple times through the scheduler.  Just
resetting for the processed instructions reduces runtime from 21s to 16s.
No change on steam shader-db runtime across several runs.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23635>