drm/amdgpu: fix lockup when clean pending fences

The first lockup fence will lock the fence list of scheduler.
Then cancel the delayed workqueues for all clean pending fences
without waiting the workqueues to finish.

Change-Id: I9bec826de1aa49d587b0662f3fb4a95333979429
Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>