Fix Plastik bottom and right border

Summary:
x and y are always relative to the parent, adding parent.width to
calculate the position doesn't make sense.

Original author meant just "x: parent.width-1"
But we may as well just use anchors. Programatically it comes out the
same.

Test Plan:
No more weird vertical lines in the KCM when view is slightly less than 2 previews wide.
Modded colour to hardcoded "red" for easier visualisation, checked it looked ok when set as a real deco.

Reviewers: #plasma

Subscribers: plasma-devel, kwin, #kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9776