drm/amdgpu: fix use-after-free during gpu recovery

[Why]
    [  754.862560] refcount_t: underflow; use-after-free.
    [  754.862898] Call Trace:
    [  754.862903]  <TASK>
    [  754.862913]  amdgpu_job_free_cb+0xc2/0xe1 [amdgpu]
    [  754.863543]  drm_sched_main.cold+0x34/0x39 [amd_sched]

[How]
    The fw_fence may be not init, check whether dma_fence_init
    is performed before job free
Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_job.c

@@ -169,7 +169,11 @@ static void amdgpu_job_free_cb(struct drm_sched_job *s_job)
 	amdgpu_sync_free(&job->sync);
 	amdgpu_sync_free(&job->sched_sync);
 
-	dma_fence_put(&job->hw_fence);
+	/* only put the hw fence if has embedded fence */
+	if (!job->hw_fence.ops)
+		kfree(job);
+	else
+		dma_fence_put(&job->hw_fence);
 }
 
 void amdgpu_job_set_gang_leader(struct amdgpu_job *job,