retry_pageflip is now dead code, since drm_connector_start_renderer
isn't called anymore. It was previously called when enabling an output.
The name "retry_pageflip" was a little confusing because the function
retried a modeset and the timer wasn't set up while performing a simple
page-flip.
Let's just remove this altogether for now. We can discuss whether it's
worth it to bring it back. Should we only do it on failed page-flips?
Should we only do it on EBUSY?