drm/i915/gvt: remove unresolved vfio pin/unpin pages interface dependency

Instead of partially depending on vfio pin/unpin pages interface if
mdev is available, which would result in failure if vfio is not
on. But replace with a wrapper which need to be fixed till mdev
support got fully merged.

Cc: Jike Song <jike.song@intel.com>
Cc: Xiaoguang Chen <xiaoguang.chen@intel.com>
Reviewed-by: Xiaoguang Chen <Xiaoguang.chen@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>