drm/nouveau: Fix double call to drm_sched_entity_fini()

drm/nouveau: Fix double call to drm_sched_entity_fini()

nouveau_abi16_chan_fini() does invoke drm_sched_entity_fini() twice:
Once directly, and a second time through nouveau_sched_destroy().

That's likely undesired behavior and might be a bug since
drm_sched_entity_fini() decrements reference counts.

Fix the issue by using the appropriate function,
drm_sched_entity_kill(), to kill all remaining jobs within the entity.

Cc: stable@kernel.vger.org
Fixes: 9a0c32d698c1 ("drm/nouveau: don't fini scheduler if not initialized")
Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20260415144956.272506-3-phasta@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>