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

V2:  edit the commit message to contain more info
The W/A spreadsheet says this is still required, but the b-spec says
it's not for BYT-T.  So the documentation is not clear.  However,
our experience with the other SKUs of BYT-I/M on Android and Linux
suggests that setting this bit actually causes GPU hang for certain
OGL benchmark applications.

Removing this bit completely resolves the GPU hangs.

Signed-off-by: Sinclair Yeh <sinclair.yeh@intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>