drm/nouveau/nvkm: fini object children in reverse order

Turns out, we're currently tearing down the disp core channel *before*
the satellite channels (wndw, etc) during suspend.

This makes RM return NV_ERR_NOT_SUPPORTED on attempting to reallocate
the core channel on resume for some reason, but we probably shouldn't
be doing it on HW either.

Tear down children in the reverse of allocation order instead.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230525003106.3853741-1-skeggsb@gmail.com