drm/amdgpu: fix and cleanup job destruction

Remove the job reference counting and just properly destroy it from a
work item which blocks on any potential running timeout handler.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Monk.Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>