drm/amdgpu:use job's list instead of check fence

because if the fence is really signaled, it could already
released so the fence pointer is a wild pointer, but if
we use job->base.node we are safe because job will not
be released untill amdgpu_job_timedout finished.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>