drm/nouveau/kms/nv50-: ensure window updates are submitted when flushing mst disables

It was possible for this to be skipped when shutting down MST streams, and
leaving the core channel interlocked with a wndw channel update that never
happens - leading to a hung display.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-By: Lyude Paul <lyude@redhat.com>