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: mesa-stable@lists.freedesktop.org Reviewed-by:Jordan Justen <jordan.l.justen@intel.com> Reviewed-by:
Kenneth Graunke <kenneth@whitecape.org> (cherry picked from commit 55314c5b)