drm/i915: Prefer setting PTE cache age to 3

drm/i915: Prefer setting PTE cache age to 3

We have conflicting benchmark data that suggest either age 0 or age 3 is
better. However, the earlier benchmark on which we based the switch to
age 0

(commit 4cfb93febfeea7f4b090c0c4a58419b473a4e257
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Thu Jul 4 11:02:03 2013 -0700

    drm/i915/hsw: Set correct Haswell PTE encodings)

actually seems to prefer the default PTE encoding as age 3. Presumably,
this is in part due to the use of MOCS to override the PTE encodings
when appropriate.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=69870
Tested-by: mengmeng.meng@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Eric Anholt <eric@anholt.net
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>