Fix non-atomic output init

Summary:
The initial blank needs to happen after the output device has been
initialized to get the current mode size.

Also we can't assert in legacy mode on the page flip pending since
a page flip can also be the result of a CRTC blank using a different.

BUG: 412684
FIXED-IN: 5.17.1

Test Plan: Full Plasma session

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D24829