drm/i915: implement WaDisableL3CacheAging on VLV

Needs to be set on every context restore as well, so set it as part of
the initial state so we can save/restore it.  Note this removes the IVB
workaround value from VLV and uses the default value, just adding in the
L3 cache aging disable bit, since the IVB value is wrong for VLV.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Antti Koskipää <antti.koskipaa@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>