Update:  Due a lot of testing, my best speculate is, that the Thinkpad pro dock 
90 w acts as MST-DP Hub ( Display Port 1.2 Hub )
After Docking the Kernel renames DP Ports, this is needed in use with real DP 
1.2 Displays. But from then ( docked in ) the resume code in DRM is not longer 
working ( it works if the displays are attached directly to the laptop ).
So the resume bug should be somewhere in the Intel DRM / MST-DP code.

After all i found some easy workaround ( not with xrandr, cause it dont works 
with more the one external  display )
and updated 
https://sys4.de/de/blog/2016/03/09/external-displays-thinkpad-pro-dock-90w-stay-switched-after-resume/

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https://bugs.launchpad.net/bugs/1555078

Title:
  external displays thinkpad pro dock 90w stay switched after resume

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