drm/i915/gvt: Add software PTE flag to mark special 64K splited entry

This add a software PTE flag on the Ignored bit of PTE. It will be used
to identify splited 64K shadow entries.

v2: fix mask definition.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>