drm/nouveau: chan: use channel class definitions

Use channel class definitions instead of magic numbers.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231002135008.10651-2-dakr@redhat.com