brw: Allow additional flags registers on Xe2+

Xe2 adds two more flags registers. We barely use the second flags
register on previous platforms, so the omission was not previously
noticed.

There are several efforts in progress that will add using of more flags
registers.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35415>
3 files changed