[Bug Report] date-picker

### Mint UI version
2.2.3

### OS/Browsers version
all

### Vue version
2.2.6

### Reproduction Link
[http://elemefe.github.io/mint-ui/#/datetime-picker](http://elemefe.github.io/mint-ui/#/datetime-picker)

### Steps to reproduce
触摸年份选项（非滑动），年份会乱跳。

### What is Expected?
有规律滑动

### What is actually happening?
触摸年份选项（非滑动），年份会乱跳。

Start with the datetime-picker reproduction link and reproduce the issue by touching a year option without swiping. Inspect the date-picker implementation responsible for year selection and verify that touching a year produces regular, non-jumping scrolling without changing the expected picker behavior.

Written by the indexing model from the issue text.