Graphics on thinkpad x270 after dock/undock works only for the first time (CPU pipe B FIFO underrun)

[Jiri Kosina]

When I dock my thinkpad x270 [1] and run xrandr --auto, image correctly 
appears on external display.

After undocking and docking again, the image on external display doesn't 
appear any more, and this pops up in dmesg

	[drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun

After restarting X (no need to reboot), I am able to dock again and get 
image on external display.

I don't think this is a regression, I tried a few older kernels up to 
something around 4.11, and nothing worked.

In case any more information is needed to debug this, please let me know, 
I'll happily provide it.

Thanks,

-- 
Jiri Kosina
SUSE Labs