Banding with 10.12.4/10.12.5 and Dell 3415W (ATI 6870)

[krachbumms]

Hi!

I have a dual screen setup, Dell U3415W and Dell U2711 on a ATI 6870.
Everything worked fine until 10.12.3.
After updating to either 10.12.4 or 10.12.5 my the U3415W shows banding,
very noticeable when the login screen fades in (looks like a 256 color display).
Switching ports doesn't help.
I attached a pic to show the banding on a gradient.
Is there a known fix for this?

 

[krachbumms]

I found the solution, you have to disable DisplayPort 1.2 in the screen settings.
Works now again like a charm.