[Bug Report] Use drawer in a router-view result in broken the animation

### Element UI version
2.13.1

### OS/Browsers version
Chrome Version 83.0.4103.97 (Official Build) (64-bit)

### Vue version
2.6.11

### Reproduction Link
https://jsfiddle.net/op458tjq/29/

### Steps to reproduce
1. click go to bar then close the drawer repeatedly
2. watch the animation.
3. sometimes after the button was just clicked, the draw showed first, animate then.

it's because el-drawer use <transition/> to implement animation. but view router will cause el-drawer to unmount and mount during an expected animation, this results in a confliction.

I write a component using plain CSS to replace el-drawer but cause a new bug that hard to maintain the z-index between my component and Element UI components.
I think we need a better implementation.

### What is Expected?
smoothly animation.

### What is actually happening?
the animation was broken by vue-router