https://bugs.freedesktop.org/show_bug.cgi?id=73530
--- Comment #82 from N.Leiten nickleiten@gmail.com --- 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.