D23600: [platforms/drm] Rework enablement and dpms switching

romangg created this revision.
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REVISION SUMMARY
  This patch further refines output management.
  
  We go now through AbstractWaylandOutput virtual functions to enable and
  disable outputs.
  
  Dpms changes and enablement switches use separate code paths at start in the
  Drm backend code since they are similar but not the directly same. Common code
  is shared though, functions are renamed accordingly.
  
  Asserts have been put in place to better understand and check the control
  flow. A seemingly unnecessary call to DrmOutput::pageFlipped on reactivation
  after Vt switch has been removed to allow for that.
  
  In future patches we need to look additionally at the legacy mode switching
  code path which was and is still not working and better handling of the
  current monitor Dpms state. For example a monitor being switched off is not
  properly acted on and the workspace still expanded.

TEST PLAN
  With one and two monitors:
  
  - Dpms off/on
  - Vt switches
  - Screen disable/enable

REPOSITORY
  R108 KWin

BRANCH
  enablementDpmsRework

REVISION DETAIL
  https://phabricator.kde.org/D23600

AFFECTED FILES
  abstract_wayland_output.cpp
  abstract_wayland_output.h
  platform.cpp
  platform.h
  plugins/platforms/drm/drm_backend.cpp
  plugins/platforms/drm/drm_backend.h
  plugins/platforms/drm/drm_output.cpp
  plugins/platforms/drm/drm_output.h

To: romangg, #kwin
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