the vGPU write on TRTTE and 0x4dfc is now write to vreg first. their
values all be restored hardware when context switching.

Fixes: e39c5add ("drm/i915/gvt: vGPU MMIO virtualization")
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>