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/kernel/kernel_types.h

@@ -1202,7 +1202,8 @@ typedef struct KernelBVH {
 	int have_curves;
 	int have_instancing;
 	int use_qbvh;
-	int pad1, pad2;
+	int use_bvh_steps;
+	int pad1;
 } KernelBVH;
 static_assert_align(KernelBVH, 16);
 

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*/,