drm/nouveau/kms/nv50-: move audio enable post-modeset

- adds tracking for post-UPDATE modeset operations, similar to mst[mo]'s
- audio won't work on RM without this
- we should probably have been doing this anyway

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Danilo Krummrich <me@dakr.org>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230919220442.202488-19-lyude@redhat.com