drm/amd/sched: NULL out the s_fence field after run_job

amd_sched_process_job drops the fence reference, so NULL out the s_fence
field before adding it as a callback to guard against accidentally using
s_fence after it may have be freed.

v2: add a clarifying comment

Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andres Rodriguez <andresx7@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>