Revert "drm/i915/gvt: Refine broken PPGTT scratch"

This reverts commit b20d09886fd1b74cd2255d846029a049e524db14.

This caused windows driver boot errors for invalid page address.
Revert for now.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>