[RFC] radeon MST support - initial pieces

[Dave Airlie]

Hey,

So I've had this work done for ages and gotten a bit stalled, it
only kind off works, but I think I'm better sending it out for review
and making getting some insight from AMD folks.

It applies on top of the auxch native patch I've send already and
Alex fixed up.

It detects my 5k monitor and sees both halves, however for some
reason one half is colored slighty different than the other, not sure
what isn't getting loaded properly, also with an MST hub and DVI adapter
I get some wierd mode timings that confuse one of my monitors at some
resolutions, again not many ideas on why that is.

Its hidden behind a radeon module option defauled to off for now.

The 27" 5K imacs looks like they really want MST support.

Dave.