drm/sched: Cancel and flush all outstanding jobs before finish.

commit e582951b

 upstream.

To avoid any possible use after free.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/414814/
CC: stable@vger.kernel.org
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>