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>

drivers/gpu/drm/i915/gvt/handlers.c

@@ -475,6 +475,7 @@ static i915_reg_t force_nonpriv_white_list[] = {
 	_MMIO(0x7704),
 	_MMIO(0x7708),
 	_MMIO(0x770c),
+	_MMIO(0x83a8),
 	_MMIO(0xb110),
 	GEN8_L3SQCREG4,//_MMIO(0xb118)
 	_MMIO(0xe100),