drm/amdgpu: Skip ring soft recovery when fence was NULL

amdgpu_ring_soft_recovery would have Call-Trace,
when s_fence->parent was NULL inside amdgpu_job_timedout.
Check fence first, as drm_sched_hw_job_reset did.

Signed-off-by: Wentao Lou <Wentao.Lou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>