Place internal osd windows according to placement policies

Currently, internal on screen display windows have the bypass window
manager hint set. If that hint is set, the osds must place themselves,
but they don't do it.

As far as I know, there is no any reason why internal OSDs have that
flag set.

By removing the Qt::BypassWindowManagerHint flag, we let kwin core
know that it's okay to place internal OSD windows.

BUG: 400675


(cherry picked from commit 4559d303)