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