[Bug Report] el-date-picker控制台警告：Instead, use a data or computed property based on the prop's value. Prop being mutated: "placement"

### Element UI version
2.15.9

### OS/Browsers version
Mac

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/elanyoung/pen/abqKxqE

### Steps to reproduce
打开控制台即可看到警告

### What is Expected?
解决控制台警告

### What is actually happening?
2.5.18版本没有此警告，更新2.5.19就出现了，placement属性造成的