There is no way to do this with a single LDAP API call. You'll always have to use one LDAP search operation to get givenname and sn attributes, and one LDAP modify operation to modify the displayName attribute.
If you use the command line ldaptools "ldapsearch" and "ldapmodify", you can do this easily with some shell scripting, but you'll have to be careful: sometimes ldapsearch(1) can return LDIF data in base64 format, with UTF-8 strings that contain characters beyond ascii. For instance: 'sn:: Base64data' (note the double ':')
So, if I were you I would use a simple script in my language of choice, that has an LDAP API, instead of using shell commands. This would save me the troubles of base64 decoding that the ldaptools sometimes impose.
For instance, with php-cli, your script would be roughly like this (perhaps some more error checking would be appropriate):
<?php
$ldap = ldap_connect('host');
ldap_bind($ldap, ...);
$sr = ldap_search($ldap, 'ou=people,...', 'objectclass=*');
$entries= ldap_get_entries($ldap, $sr);
for($i=0; $i<$entries['count']; $i++) {
$modify = array('displayname' => $entries[$i]['givenname'] . ' ' . $entries[$i]['sn']);
ldap_modify($ldap, $entries[$i]['dn'], $modify);
}
Addendum: if you want to keep this data up to date without any intervention, you will probably need to use a specialized OpenLDAP module that keeps "virtual" attributes, or even a virtual directory, such as Penrose or Oracle Virtual Directory, on top of OpenLDAP. However this might be overkill for a simple concatenation of attributes.