[wayland] Make sure fullscreen windows aren't repositioned

Summary:
on X11, setFullScreen is always called after Placement::place()
so they always have the correct geometry.
on wayland, the window if is shown directly as fullscren,
is set fullscreen in init() then place() is evecuted, potentially
moving it to a wrong position and potentially even size (which happens
with maximiziong placement strategy)
so instead of place() the client needs to be explicitly set at
fullscreen geometry

Test Plan:
fullscreen windows always appear with the proper geometry
autotests still pass

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24542