the drm-intel-nightly kernel didn't fix the issue.

I began attempting to trace the issue and search for clues as to why
this is happening.

I am attaching three kernel log files with kernel switch: drm.debug=0x05

All of these are kernel logs after a "xset dpms force off".
- "success" is with the screen resumed successfully after a few seconds of 
being suspended
- "fail-proper" screen resumed after ~20 seconds, and the issue occurs
- "fail-reschg" after about 15 seconds, the computer wakes itself with no user 
input, external monitors are switched off, and the panel (eDP) drops to 
1680x1050 and cannot be set to a higher resolution until a "service lightdm 
restart"

I noticed the existence of errors around "set_hdr_from_dst_qlock" in
fail-proper only and came across two patches to drm_dp_mst_topology.c -
I applied these four patches and recompiled the kernel module, and the
problem still exists.

http://www.spinics.net/lists/stable/msg113900.html

After this patch it seems like suspending does one of three behaviors:
- Wakes itself after 15 seconds with no issues
- Wakes itself after 15 seconds and the externals detach, and laptop panel goes 
to a lower resolution
- Doesn't wake, but on user input returns with the two external screens still 
part of the desktop, but with no video

** Attachment added: "verboselogs.tar.gz"
   
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1527829/+attachment/4549545/+files/verboselogs.tar.gz

** Changed in: linux (Ubuntu)
       Status: Incomplete => Confirmed

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https://bugs.launchpad.net/bugs/1527829

Title:
  displayport MST external monitors don't return from sleep mode

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