Avoid calling child component method in noteable_note.vue

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24983#note_139681794:

noteable_note.vue calls resetAutoSave which is defined in the child component: https://gitlab.com/gitlab-org/gitlab-ce/blob/e927833b941122f25252712bc68b37041b38ba2c/app/assets/javascripts/notes/components/noteable_note.vue#L205

This means rather than the child component listening to the parent state, the parent mutates the state of the child directly which is against the way how Vue works and we need to fix it: https://vuejs.org/v2/guide/components-props.html#One-Way-Data-Flow