drm/i915 mst support

[Dave Airlie]

Another round of the MST support for i915 haswell.

This relies on the aux locking and i915 irq rework patches I posted
already.

this also splits out some more i915 rework into earlier patches.

The main fix is not talking to devices if HPD isn't asserted, at
least on the dock I have it will reply to DPCD without HPD being
asserted but it just confuses everyone.

Otherwise it contains a bunch of stability fixes, and avoids
the worst backtraces of i915 state tracker, which is fine when hw
is involved but MST doesn't really fit so well with it, esp when
you hot unplug some hardware, and the state tracker expects it to
remain in the same state until userspace catches up, (note the hw
is gone!).

All available on:
git://people.freedesktop.org/~airlied/linux drm-i915-mst-support

Dave.