Cycles: Pass special flag whether BVH motion steps are used

Doesn't currently change anything, but would need for some future
work here.

It uses existing padding in kernel BVH structure, so there is
nothing changed memory-wise.

intern/cycles/render/mesh.cpp

@@ -1880,6 +1880,7 @@ void MeshManager::device_update_bvh(Device *device, DeviceScene *dscene, Scene *
 
 	dscene->data.bvh.root = pack.root_index;
 	dscene->data.bvh.use_qbvh = scene->params.use_qbvh;
+	dscene->data.bvh.use_bvh_steps = (scene->params.num_bvh_time_steps != 0);
 }
 
 void MeshManager::device_update_flags(Device * /*device*/,