This is consistent with what all other roles do. If someone includes a
role, the assumption is that they want its functionality enabled.

The playbook distribution then disables components via
`group_vars/matrix_servers`. We've always had `matrix_grafana_enabled: false`
there, so flipping the in-role `_enabled` flag to `true` does not change
anything for playbook users. Users who import the roles individually in
their own other playbooks (and who don't use `group_vars/matrix_servers`)
may observe a change in the defaults with this.