backends/drm: support link-status
Userspace is expected to reload modes, do a modeset and set link-status to good again, if link-status gets bad. This is needed to prevent some black screen situations.
As we're already detecting modelist changes every time udev signals something to us, all that needs to be done is to do a modeset and set the property to good again. With legacy a modeset automatically sets it to good (no idea why it doesn't with ams) so that's dealt with, too.