[platforms/x11] Fix valigrind warning copying window icon for windowed mode

Summary:
windowIcon.pixmap().bits() creates and deletes the QPixmap object.

Which means data is dangling when we call m_winInfo->setIcon()

Surprisingly harmless in real life.

Test Plan: Reran kwin_wayland on X11 with valigrind

Reviewers: KWin, apol

Reviewed By: apol

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D28667