kdb: handle enterprise principal lookup in AS_REQ
    
    Refactoring of the get_principal() code in commit
    b5876f30d4000424cc8122498c411f812b3a0959 broke handling of enterprise
    principal lookup for AS request (kinit -E user@ipa.test@IPA.TEST).
    
    Related: https://pagure.io/freeipa/issue/8319
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>