drm/scheduler: fix fence ref counting

We leaked dependency fences when processes were beeing killed.

Additional to that grab a reference to the last scheduled fence.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220929180151.139751-1-christian.koenig@amd.com