drm/i915/gvt: fix wrong offset when loading RCS mocs

Fix the wrong offset of the RCS specific mocs

Fixes: 17865713 ("drm/i915/gvt: vGPU context switch")

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>