drm/i915: Do not set L3-LLC Coherency bit in ctx descriptor

According to Spec this is a reserved bit for Gen9+ and should not be set.

Change-Id: I0215fb7057b94139b7a2f90ecc7a0201c0c93ad4
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>