[Bug Report] [Colorpicker] The predefined color which is checked before clear can not checked after clear

### Element UI version
2.15.1

### OS/Browsers version
window7 chrome 69.0.3497.81

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/charmkun/pen/abpbMmJ

### Steps to reproduce
1. Trigger the colorpicker with predefine colors. Pick up a predefined color.
2. Click on the clear button (then colorpicker panel closed).
3. Trigger the colorpicker again and pick the same predefined color. Color can not be checked.

### What is Expected?
All the colors can be checked after clear.

### What is actually happening?
The color checked last time can not be checked after clear.