[Bug Report] draggable cannot get indexOf items

### Element UI version
2.13.2

### OS/Browsers version
Windows / Chrome 84

### Vue version
2.5.17

### Reproduction Link
https://jsfiddle.net/robertnicjoo/fpt5aqyu/5/

### Steps to reproduce
Elements can't be dragged due to `allowDrag` function which can't get items index number.

### What is Expected?
Ability to drag items regardless of their position (i.e parent can be move to be children of other items, or children be moved as other parents child etc. `free to move items`)

### What is actually happening?
items can't be moved