drm/i915/gvt: add define GEN9_MOCS_SIZE

No functional change. This defination will also be used in future patchesi.

v4:
- refine patch description (Kevin)

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>