D23535: [server] Use cached current mode more and assert validness

[David Edmundson]

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

REPOSITORY
  R127 KWayland

BRANCH
  useCachedMode

REVISION DETAIL
  https://phabricator.kde.org/D23535

To: romangg, #kwin, davidedmundson
Cc: kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns