vp9: SVC: Modify condition to allow for copy partition.
vp9: SVC: Modify condition to allow for copy partition.

When temporal layers are used, only allow for copy partition
on the top temporal enhancement layer frames.

Change-Id: I5472abdc0f9f6c8dafa75a7a84c615e08ae22af8

ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx