[Bug Report] 如果有两个el-button通过v-if:flag判断显示，并异步修改这个flag的值，如果第二个button有disabled属性就会无效

### Element UI version
2.15.9

### OS/Browsers version
Windows

### Vue version
2.6.11

### Reproduction Link
https://codesandbox.io/s/infallible-water-frjc5q?file=/src/App.vue

### Steps to reproduce

import ElButton from 'element-ui/packages/button'
import ElInput from 'element-ui/packages/input'
export default {
components: {
ElInput,
ElButton,
},
data() {
return {
info: '',
}
},
created() {
setTimeout(() => {
this.info= {}
}, 500)
},
}

### What is Expected?
预期应该和el-input一样，el-button查看是disabled='true'

### What is actually happening?
el-button查看是disabled=‘false’ ，如果把el-button的计算属性buttonDisabled改为和el-input一样就没有问题，我想问下为什么现在的版本要用this.$options.propsData.hasOwnProperty('disabled')判断