[Bug Report] el-date-picker got a problem

### Element UI version
2.15.9

### OS/Browsers version
macOS Big Sur

### Vue version
2.7.0-beta.5

### Reproduction Link
https://elementui.github.io/issue-generator/#/zh-CN

### Steps to reproduce
just copy the same code form element-ui document

### What is Expected?
I am not pass the placement props to the component

### What is actually happening?
vue.esm.js?a026:628 [Vue warn]: Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value. Prop being mutated: "placement"

found in

---> at packages/date-picker/src/picker.vue
at packages/form/src/form-item.vue
at packages/form/src/form.vue

at node_modules/zx-layouts/VabQueryForm/VabQueryFormRightPanel.vue

at node_modules/zx-layouts/VabQueryForm/index.vue
at src/views/business/activity/index.vue
at src/layouts/components/VabAppMain/index.vue
at src/layouts/index.vue
at src/App.vue