Fix placement of KSplash

Summary:
In a bug report there was a reference that on multi-screen KSplash is
not placed correctly. I investigated and noticed that it is an OSD which
sets an own position. In KWin the events were processed correctly but
the position was off.

The problem is that KWin has code to correct the position of an OSD when
it's size changes. This happens also on first damage and then the window
gets incorrectly placed when the position is set. So honor that the
position is set.

Test Plan: Restarted the session, ksplash positioned correctly now.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D8268