intel: Let drivers call brw_nir_lower_cs_intrinsics()

intel: Let drivers call brw_nir_lower_cs_intrinsics()

The motivating factor is: this lowering may cause
nir_intrinsic_load_local_group_size intrinsics to be added to the
shader, and by moving this around we make possible for the drivers to
lower that intrinsic by themselves.

Iris will do just that in a later patch for implementing variable
group size.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4794>