D20521: Don't rescale every output every time outputs change

apol created this revision.
apol added a reviewer: Plasma.
Herald added a project: KWin.
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REVISION SUMMARY
  Every time a display gets connected/disconnected, every output will be
  reinitialized. We don't really need to reset the scaling to 1 every time
  we don't know, especially since the setting has been set voluntarily.

TEST PLAN
  When I disconnect the external display, my laptop doesn't fallback to scale=1.

REPOSITORY
  R108 KWin

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D20521

AFFECTED FILES
  abstract_output.h
  plugins/platforms/drm/drm_backend.cpp

To: apol, #plasma
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