Fix updating layer when setting a ShellClient to fullscreen

Summary:
The call to update the layer was performed before adjusting to
fullscreen. Thus the layer didn't get updated at all as it still
evaluated to "not fullscreen".

Reviewers: KWin, Plasma on Wayland, bshah

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2758