Displayport MST (1.2) does not work correctly.

Displayport MST (1.2) does not work correctly.

Expected behaviour
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When Displayport 1.2 functionality is enabled on two monitors and are connected in daisychain fashion to a displayport on a system running Ubuntu 17.04, Ubuntu should be able to use the displays as a single large X display

Actual Behaviour
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Both displays, including the laptop display are blank or are in power save mode. Or one display is active, and the other non functional. Or the laptop display is visible but the external displays are non functional.

Steps to Reproduce
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1. Enable the NVIDIA GPU on a laptop with Displayport 1.2 support (GPU used in test, NVIDIA Corporation GM107GLM [Quadro M2000M] (rev a2))
2. Enable Displayport 1.2 support on two external monitors (monitors used in test 2x DELL 2715H)
3. Connect the monitors in daisy chain fashion

Workaround
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Disable Displayport 1.2 and connect one monitor via HDMI and one via Displayport.

Notes
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Both displays are detected and appear in the Nvidia Server Settings. Daisychaining functions as expected on the same system using Windows. Google reveals similar problems have been reported and a potential workaround using `xset dpms force off` to reset power management. This was not observed to work using the test system.

GPU - NVIDIA Corporation GM107GLM [Quadro M2000M] (rev a2)
Driver Version - 381.22
Ubuntu 17.04
Kernel 4.4.0-93-generic