drm/i915: Enable GTT caching on gen8

GTT caching was disabled by default on gen8 due to not working with
big pages. Some information suggests that it got fixed, but still
GTT caching has been left disabled by default. Or could be it just
meant that the default was changed to off, and hence the problem
got solved.

Enable GTT caching in the hopes of some performance increase.
Whether or not the big pages issue has been fixed is irrelevant
at this stage since we don't use big pages.

This gives me a 1-2% improvement in xonotic on my BSW. Haven't tried
BDW, but supposedly it has larger TLBs so might not benefit as much.
On HSW GTT caching is enabled by default.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>