[Bug Report] el-tooltip bug, popper won't show correctly when "enterable" attribute is false

### Element UI version
2.13.1

### OS/Browsers version
macos/chome 81

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/zc-1028/pen/MWazaVv

### Steps to reproduce
firstly, move to the text, secondly, move to the popper and then move to the text again quickly

### What is Expected?
the popper will show again

### What is actually happening?
the popper won't show again