D25867: [platforms/drm] Do plane and object chore

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REVISION SUMMARY
  This cleans up some of the code, moves and deletes superfluous functions,
  improves in-code docs and runtime warnings.

TEST PLAN
  On vt.

REPOSITORY
  R108 KWin

BRANCH
  drm-chore

REVISION DETAIL
  https://phabricator.kde.org/D25867

AFFECTED FILES
  plugins/platforms/drm/drm_object.cpp
  plugins/platforms/drm/drm_object.h
  plugins/platforms/drm/drm_object_plane.cpp
  plugins/platforms/drm/drm_object_plane.h

To: romangg, #kwin
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