[Bug Report] cascader cannot set default values after setting lazy and multiple at the same time

### Element UI version
2.12.0

### OS/Browsers version
Windows 10/Chrome63.0.3239.132

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/oq0j1ram/2/

### Steps to reproduce
```

```
```
data() {
var id = 0
return {
defaultValue: [[1, 2, 5], [1, 2, 3]],
props: {
lazy: true,
multiple: true,
lazyLoad(node, resolve) {
const { level } = node;
setTimeout(() => {
const nodes = Array.from({ length: level 1 }).map(item => ({
value: id,
label: `选项${id}`,
leaf: level >= 2
}));
// 通过调用resolve将子节点数据返回，通知组件数据加载完成
resolve(nodes);
}, 1000);
}
}
}
}
```
无法回显默认值

### What is Expected?
可以回显默认值，至少做到手动点击懒加载后默认值被选中

### What is actually happening?
无法回显默认值