Display doesn't power on/off properly

Configuration: Ubuntu 16.04, xfce, two HP EliteDisplay E232, displayport, Nvidia GTX960.

Problems:

• Displays don't enter sleep mode.
• If displays are turned off, they are lost by the Ubuntu system and the computer will need to be rebooted.

Some pages:

• http://askubuntu.com/questions/546818/displayport-monitor-not-detected-if-switched-off-and-on-again 
• https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers-331/+bug/1308105/comments/115 
• http://bug-attachment.xfce.org/attachment.cgi?id=6590 
• https://bugzilla.xfce.org/show_bug.cgi?id=11107#c53 

The bug fix  mentioned partly solves the issue with getting the displays working again after being powered off.