drm/i915/gvt: Clear the shadow page table entry after post-sync

A shadow page table entry needs to be cleared after being set as
post-sync. This patch fixes the recent error reported in Win7-32 test.

Fixes: 2707e444

 ("drm/i915/gvt: vGPU graphics memory virtualization")
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>