Fix 'Add to Dashboard' issue with builtins

Summary: Ref T5307. Actually check the built in query with query, not engine.

Test Plan: Try a builtin query, and a custom query when making a dashboard panel.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T5307

Differential Revision: https://secure.phabricator.com/D17521