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>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index e135182..f471d21 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -284,6 +284,7 @@
 		struct drm_mm gtt_space;
 
 		struct io_mapping *gtt_mapping;
+		int gtt_mtrr;
 
 		/**
 		 * List of objects currently involved in rendering from the