[RFC] DisplayPort MST support

Hey,

So this set is pretty close to what I think we should be merging initially,

Since the last set, it makes fbcon and suspend/resume work a lot better,

I've also fixed a couple of bugs in -intel that make things work a lot better.

I've bashed on this a bit using kms-flip from intel-gpu-tools, hacked to add 3 monitor support.

It still generates a fair few i915 state checker backtraces, and some of them are fairly hard to work out, it might be we should just tone down the state checker for encoders/connectors with no actual hw backing them.

Dave.