drm/sched: Avoid data corruptions

Wait for all dependencies of a job  to complete before
killing it to avoid data corruptions.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210519141407.88444-1-andrey.grodzovsky@amd.com