drm/i915/gvt: update force-to-nonpriv register whitelist

Host print below warning message when creating guest:
"gvt: vgpu(2) Invalid FORCE_NONPRIV write 83a8".

Register 0x83a8 should be in force-to-nonpriv whitelist as required by
guest

v2: update commit message to describe purpose of this patch in detail
(zhenyu wang)

Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>