D29868: [drm] Do not blank crtc in atomic mode

jackie created this revision.
jackie added reviewers: scao, zzag.
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REVISION SUMMARY
  In  atomic setting mode, when external display tears down, it will blank crtc with dump buffer by legacy api, then re-plug the external display, the blank can not remove. We think the blank is not needed in atomic mode setting.

REVISION DETAIL
  https://phabricator.kde.org/D29868

AFFECTED FILES
  plugins/platforms/drm/drm_object_crtc.cpp

To: jackie, scao, zzag
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