drm/nouveau/fifo: return ERR_PTR from nvkm_runl_new()

Callers expect this - not NULL.

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/20230525003106.3853741-6-skeggsb@gmail.com