drm/i915/gt: Add support of mocs propagation

Now there are lots of Command and registers that require mocs index
programming.
So propagating mocs_index from mocs to gt so that it can be
used directly without having platform-specific checks.

V2:
Changed 'i915_mocs_index_gt' to anonymous structure.

Cc: CQ Tang<cq.tang@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ayaz A Siddiqui <ayaz.siddiqui@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210903092153.535736-2-ayaz.siddiqui@intel.com