[Bug Report] DateTimePicker editable property does not disable in modal input fields

### Element UI version
2.13.0

### OS/Browsers version
Mac OS 10.15.4 / Chrome 80.0.3987.149

### Vue version
2.6.11

### Reproduction Link
https://jsfiddle.net/hxp90jbg/

### Steps to reproduce
1) Create a date picker component with type "datetime"
2) Add :editable="false".

### What is Expected?
If you set editable to false, one would think that all input fields are not editable and only UI clicking works.

### What is actually happening?
Only the main input field is disabled from editing, inside the modal the fields are editable.