drm/amdgpu: add reference for **fence

fix fence is released when pass to **fence sometimes.
add reference for it.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian K?nig <christian.koenig@amd.com>
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
index f428288..8796938 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
@@ -136,6 +136,7 @@
 		sched_job->free_job(sched_job);
 	mutex_unlock(&sched_job->job_lock);
 	/* after processing job, free memory */
+	fence_put(&sched_job->s_fence->base);
 	kfree(sched_job);
 }
 struct amdgpu_cs_parser *amdgpu_cs_parser_create(struct amdgpu_device *adev,