intel/elk: Move wpos_w setup right into nir_intrinsic_load_frag_w.

Given that the intrinsic will be CSEed at the NIR level, we don't need to
preemptively set it up at the top of the shader.  No change in HSW shader-db.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>