D8210: Use KWindowSystem's own platform detection API instead of duplicated code

[Kai Uwe Broulik]

broulik accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R278 KWindowSystem

BRANCH
  isplatform-internal

REVISION DETAIL
  https://phabricator.kde.org/D8210

To: graesslin, #frameworks, #kwin, broulik