According to GFX PRM on 01.org, bit 31:16 of mmio 0x22028 should be masks.

Fixes: 17865713 ("drm/i915/gvt: vGPU context switch")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>