radv: Fix multi-layer blits.

We did not set the layer correctly for the dst, as we would keep
using the base layer. Same for the source image.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102710


CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit b42e106d)