drm/i915: Set up an MTRR covering the GTT at driver load.

We'd love to just be using PAT, but even on chips with PAT it gets disabled
sometimes due to an errata.  It would probably be better to have pat_enabled
exported and only bother with this when !pat_enabled.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>