Fix crash when using kwin on windowed mode

Summary:
Used to get:
kwin_wayland: kwin/composite.cpp:646: void KWin::Compositor::aboutToSwapBuffers(): Assertion "!m_bufferSwapPending' failed."

Test Plan: Ran kwin_wayland --socket dave --output-count 2 konsole

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D20056