intel/compiler: Make brw_nir_lower_intrinsics compute-specific

intel/compiler: Make brw_nir_lower_intrinsics compute-specific

It's already only ever called from brw_compile_cs and only handles
compute intrinsics.  Let's just make it CS-specific.  We can always
make it handle other stages again later if we want.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>