Revert "drm/nouveau: Remove waitque for sched teardown"

Revert "drm/nouveau: Remove waitque for sched teardown"

This reverts:

commit bead88002227 ("drm/nouveau: Remove waitque for sched teardown")
commit 5f46f5c7af8c ("drm/nouveau: Add new callback for scheduler teardown")

from the drm/sched teardown leak fix series:

https://lore.kernel.org/dri-devel/20250710125412.128476-2-phasta@kernel.org/

The aforementioned series removed a blocking waitqueue from
nouveau_sched_fini(). It was mistakenly assumed that this waitqueue only
prevents jobs from leaking, which the series fixed.

The waitqueue, however, also guarantees that all VM_BIND related jobs
are finished in order, cleaning up mappings in the GPU's MMU. These jobs
must be executed sequentially. Without the waitqueue, this is no longer
guaranteed, because entity and scheduler teardown can race with each
other.

Revert all patches related to the waitqueue removal.

Fixes: bead88002227 ("drm/nouveau: Remove waitque for sched teardown")
Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250901083107.10206-2-phasta@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>