Make panels decide async rendering nonrandomly

Summary: Ref T4986. The random rule was useful for making sure stuff works, but it works now.

Test Plan: Loaded some dashboards, got consistent async vs non-async.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9281