Comment # 82 on bug 73530 from N.Leiten

Yes, it seems this bug https://bugs.freedesktop.org/show_bug.cgi?id=90320 is
connected with our BUG.

As for this moment, no changes in SS and DPMS_ON/DPMS_OFF needed to get working
display. It is only a matter of DPCD elements was broken at some point. 

Digging further.