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