drm/amdgpu: stop blocking for page filp fences

Just register an callback and reschedule the work item if necessary.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu.h b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
index 588e86c..fc995b4 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu.h
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
@@ -761,6 +761,7 @@
 	struct fence			*excl;
 	unsigned			shared_count;
 	struct fence			**shared;
+	struct fence_cb			cb;
 };