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.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10612

Differential Revision: https://secure.phabricator.com/D17349