[bug report] after the cascade selector of cascade is assigned twice, the input display is correct, but there is a problem with the echo of cascade selector

### Element UI version
2.13.0

### OS/Browsers version
mac,windows

### Vue version
2.5.16

### Reproduction Link
https://github.com/wilsonlv

### Steps to reproduce
1、对v-model绑定的数组的首次赋值，input和CascaderPanel都么有问题
2、对v-model绑定的数组的重新赋值为空数组，然后对数组赋值，input显示正确，但是CascaderPanel还是会显示上一次的部分数据

### What is Expected?
CascaderPanel显示的和input一致

### What is actually happening?
CascaderPanel显示的和input不一致