Bug#788400: In my case it's:

[Yaroslav Molochko]

for i in /sys/class/drm/*/status; do echo $i; cat $i; done
/sys/class/drm/card0-DP-1/status
unknown
/sys/class/drm/card0-DP-2/status
unknown
/sys/class/drm/card0-eDP-1/status
unknown
/sys/class/drm/card0-HDMI-A-1/status
unknown
/sys/class/drm/card0-HDMI-A-2/status
unknown