i965: Use NIR to lower legacy userclipping.

i965: Use NIR to lower legacy userclipping.

This allows us to drop legacy userclip plane handling in both the vec4
and FS backends, and simplifies a few interfaces.

v2 (Jason Ekstrand):
 - Move brw_nir_lower_legacy_clipping to brw_nir_uniforms.cpp because
   it's i965-specific.
 - Handle adding the params in brw_nir_lower_legacy_clipping
 - Call brw_nir_lower_legacy_clipping from brw_codegen_vs_prog

Co-authored-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>