drm/i915/gt: Set unused mocs entry to follow PTE on tgl as on all others

Be consistent in our mocs setup on Tigerlake and set the unused control
value to follow the PTE entry as we previously have done. The unused
values are beyond the defines of the ABI, the consistency simplifies our
checking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112223600.30993-1-chris@chris-wilson.co.uk