[Bug Report] When dynamically set the disabled attribute of cascader's options, the children items of options will be disabled as expected, while the first-floor items are still abled.

### Element UI version
2.13.1

### OS/Browsers version
chrome 81.0.4044.129

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/azurecho/pen/VwvWbaG

### Steps to reproduce
1. selected one item of cascader (and the handleChange() method will set other first-floor item to be disabled)
2. then re-check the cascader, you'll find other first-floor items are still abled, while their children are correctly disabled

### What is Expected?
the first-floor items should also be set disabled

### What is actually happening?
the first-floor items are still abled