drm/nouveau/nvkm: use list_add_tail() when building object tree

Fixes resume from hibernate failing on (at least) TU102, where cursor
channel init failed due to being performed before the core channel.

Not solid idea why suspend-to-ram worked, but, presumably HW being in
an entirely clean state has something to do with it.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>