st/mesa: Enable clip planes lowering for geometry shaders
This is needed for the D3D12 driver, and I suspect we also need this for Zink.
/cc @zmike: Thoughts? I have a feeling the resolution with a79ca675 isn't perfect, and we might want to call nir_lower_clip_disable
inside the lower_ucp
-helper instead, but I left it this way until I have some clearer indication, as it was closer to the original patch from @lfrb.