[REGRESSION] i915: failure to see Dell 30" monitor connected to a Lenovo Haswell docking station

[Theodore Ts'o]

I recently upgraded to v3.17-rc3, and on my Lenovo T540p, I can no
longer see the my Dell 30" monitor when it is connected via the
docking station using a Displayport connector.  This worked using 3.16
kernel.

If I connect to the monitor using the mini-display, by passing the
docking station, things work fine (but of course it's annoying not to
be able to use the docking station).

Is this a known problem?  This is not the first time that we've had
regressions with this docking station.   It's vaguely reminsicent of 

	https://bugs.freedesktop.org/show_bug.cgi?id=71267

Except the system isn't hanging; it's just not seeing the monitor at all.

Thanks,

					- Ted