[Bug Report] el-input's preffix is displayed at wrong position when used without el-form tag.

### Element UI version
2.13.0

### OS/Browsers version
macOS Catalina / Google Chrome 81

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/POPOPON/pen/rNOYWNB?editors=1010

### Steps to reproduce
Use el-input component with prefix slot without el-form and el-form-item tags surrounded it.

### What is Expected?
Prefix and suffix should be centered even if it is not in el-form tags

### What is actually happening?
They are displayed at the top of the input.