fix(VCombobox): re-compute menu items before setting index

[KaelWD]

fixes #12567

Markup:

<template>
  <v-app>
    <v-container>
      <v-combobox :items="items"></v-combobox>
      <v-autocomplete :items="items"></v-autocomplete>
      <v-select :items="items"></v-select>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    data: () => ({
      items: ['a', 'b', 'c', 'd', 'e', 'f'],
    }),
  }
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)