drm/sched: Rename drm_sched_get_cleanup_job to be more descriptive

"Get cleanup job" makes it sound like helper is returning a job which will
execute some cleanup, or something, while the kerneldoc itself accurately
says "fetch the next _finished_ job". So lets rename the helper to be self
documenting.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Luben Tuikov <ltuikov89@gmail.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231102105538.391648-2-tvrtko.ursulin@linux.intel.com
Reviewed-by: Luben Tuikov <ltuikov89@gmail.com>
Signed-off-by: Luben Tuikov <ltuikov89@gmail.com>