intel: Use a system value for gl_FragCoord

intel: Use a system value for gl_FragCoord

It's kind-of an anomaly that the Intel drivers are still treating
gl_FragCoord as an input.  It also makes zero sense because we have to
special-case it in the back-end.

Because ANV is the only user of nir_lower_wpos_center, we go ahead and
just update it to look for nir_intrinsic_load_frag_coord as part of this
patch.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>