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Handle LINK_STATUS in DRM backend

The LINK_STATUS connector property is set to BAD when something bad happened to the link. Sometimes this is set when it needs retraining; often it is an indicator that the modeset couldn't actually complete (e.g. a marginal cable preventing higher-frequency modes from locking and training).

We need to:

  • check the LINK_STATUS connector property whenever we get hotplug notifications
  • if LINK_STATUS is BAD, try setting the current mode again whilst resetting the property to GOOD
  • if we get another failure, blacklist the current mode and step down one
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