How to optimize query with order by
Using an index consisting of the columns display_name, id, active and is_company would enable the query to be run using the index alone.  Ordering the columns in the order provided would remove the sort requirement.  This comes at a slightly update cost, and could replace the ix_displayname index.

If active were rare, it would be faster to order the columns active, display_name, id, 'is_company.  However, updating the active column would have higher update costs.  

You may need to select the display_name in an inner query, or use an index hint.