drm/nouveau: Add new callback for scheduler teardown

drm/nouveau: Add new callback for scheduler teardown

There is a new callback for always tearing the scheduler down in a
leak-free, deadlock-free manner.

Port Nouveau as its first user by providing the scheduler with a
callback that ensures the fence context gets killed in drm_sched_fini().

Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250710125412.128476-9-phasta@kernel.org