drm: Don't free jobs in wait_event_interruptible()

drm_sched_cleanup_jobs() attempts to free finished jobs, however because
it is called as the condition of wait_event_interruptible() it must not
sleep. Unfortunately some free callbacks (notably for Panfrost) do sleep.

Instead let's rename drm_sched_cleanup_jobs() to
drm_sched_get_cleanup_job() and simply return a job for processing if
there is one. The caller can then call the free_job() callback outside
the wait_event_interruptible() where sleeping is possible before
re-checking and returning to sleep if necessary.

Tested-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Fixes: 5918045c

 ("drm/scheduler: rework job destruction")
Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/337652/