drm/i915/gvt: not to touch undefined MOCS registers

Some engines are not available for all Gens. eg, Gen11 introduced
VCS3/VCS4/VECS2, and VCS2 is not supported on some Gen9 machines. So need to
add check before access them.

Signed-off-by: Xinyun Liu <xinyun.liu@intel.com>
Signed-off-by: Yakui Zhao <Yakui.Zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>