i965/skl/gt4: Fix URB programming restriction.
i965/skl/gt4: Fix URB programming restriction.

The comment in the code details the restriction. Thanks to Ken for having a very
helpful conversation with me, and spotting the blurb in the link I sent him :P.

There are still stability problems for me on GT4, but this definitely helps with
some of the failures.

v2: Comment fixes

Cc: 59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org
Reviewed-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler