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 0d8ff15e
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>