Fix duplicating panel when editing in column 2
Summary: Fixes T10612. We're writing a new panel to any dashboard even if it already exists. No need when just updating a panel title.
Test Plan: Add "welcome" panel to column 2 of a clean dashboard. Edit title, save. See correct panel in correct place.
Reviewed By: epriestley
Maniphest Tasks: T10612
Differential Revision: https://secure.phabricator.com/D17349