Fractional scaling in DRM kscreen integration

Summary:
The kwin code was written to treat scale as non integers, we just need
to switch to the relevant float version of methods.

Test Plan:
Used kscreen-doctor to change this
Konsole resized as appropriate
Plasmashell was all broken, but that's expected until QScreen::size is based on the
xdg-output virtual size

Reviewers: KWin, mart, graesslin, romangg

Reviewed By: KWin, mart, graesslin

Subscribers: romangg, zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13616