drm/amd/sched: move adding finish callback to amd_sched_job_begin

The finish callback is responsible for removing the job from the ring
mirror list, among other things. It makes sense to add it as callback
in the place where the job is added to the ring mirror list.

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>