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 0d8ff15e9a15f2b393e53337a107b7a1e5919b6d
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>