drm/i915: Revert workaround for disabling L3 cache aging on IVB

In commit e4e0c058
Author: Eugeni Dodonov <eugeni.dodonov@intel.com>
Date:   Wed Feb 8 12:53:50 2012 -0800

    drm/i915: gen7: Implement an L3 caching workaround.

the L3 cache aging was disabled. This was part of a shotgun response
to a number of GPU hang bugs, but there appears to be no documentation
to suggest that disabling the L3 cache age was ever required (to prevent
the GPU hangs).

Restoring the L3 cache age is a minor performance win of around 2%
on IVB:GT2. (Note that this value seems to be consistent across a number
of tests and so appears to be above the usual noise.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>