vc4: Handle nir_intrinsic_load_user_clip_plane as a vec4.

vc4: Handle nir_intrinsic_load_user_clip_plane as a vec4.

I liked having all my NIR be scalar, but nir_validate() complains that the
intrinsic writes 4 components but the destination we set up was only 1
component.  I could generate a new scalar variant, but it's a lot easier
to just leave it as a vec4.  This doesn't hurt codegen since we GC unused
uniforms, and UCP dot products use all the components anyway.