drm/sched: Fix fence reference count leak

The last_scheduled fence leaks when an entity is being killed and adding
the cleanup callback fails.

Decrement the reference count of prev when dma_fence_add_callback()
fails, ensuring proper balance.

Cc: stable@vger.kernel.org	# v6.2+
[phasta: add git tag info for stable kernel]
Fixes: 2fdb8a8f ("drm/scheduler: rework entity flush, kill and fini")
Signed-off-by: qianyi liu <liuqianyi125@gmail.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311060251.4041101-1-liuqianyi125@gmail.com