D23493: [server] Cache current mode

[Roman Gilg]

romangg added a task: T11459: AbstractWaylandOutput and output device 
consolidation.

REPOSITORY
  R127 KWayland

REVISION DETAIL
  https://phabricator.kde.org/D23493

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