anv: Enable the guardband clip test

anv: Enable the guardband clip test

In workloads where there is a lot of geometry drawn that crosses over
the edge of the viewport, this should substantially improve clipper
performance.  Not really sure why it's taken 3 years to turn it on but
we never got around to it.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>