[Bug Report] Notification 组件定位错误

### Element UI version
2.13.1

### OS/Browsers version
windows 10/chrome 81.0.4044.122

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/jxh/pen/rNOzaQx

### Steps to reproduce
在同一个语句块中，连续调用两次 this.$notify(...)
like: {
...
this.$notify(...);
this.$notify(...);
...
}

### What is Expected?
正确的定位，不会挡住上一条nofify

### What is actually happening?
挡住了上一条notify，未正确定位