vp9 denoiser: force copy block when last not a reference.
vp9 denoiser: force copy block when last not a reference.

Last reference doesn't always exist when SVC layers changed dynamically.

When last is not a reference for current layer, copy block directly on
denoiser.

Change-Id: I9d98c4d6fdcfa25ba707db3333712761b5cf9ab8

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