[bug report] when the element under app uses position: fixed layout, the mask of dialog will be displayed above the dialog

### Element UI version
2.13.0

### OS/Browsers version
win10/chrome79

### Vue version
2.6.11

### Reproduction Link
https://jsfiddle.net/pdnoqjby/1/

### Steps to reproduce
点击【点击打开 Dialog】按钮

### What is Expected?
遮罩在dialog之下，内容之上

### What is actually happening?
遮罩盖住了dialog