DPCD/AUX locking

[Dave Airlie]

So now I've been playing with MST I think get the feeling I might need
some explicit locking on the AUX channel, I think at the moment the
mode_config mutex implicitly defends the aux channel as the only real
paths into it are

a) from userspace connector probing,
b) from HPD irqs,

currently both of these on i915 at least take mode config,

however with MST I can't use mode_config for this, so I'm wondering if
I should be adding some explicit locking in the helpers or make it the
drivers problem to lock around helper access?

Any ideas, it would most likely have to be a mutex since DPCD can in
theory sleep.

Dave.