For a long time now, I don't recall which 3.x broke this but there's been many
changes since and now, when the GPU does randomly reset, the display goes
blank, X display is restored, but I can only move mouse, cannot interact with
X.

Can someone point me to some debugging approaches to see if there's some lock
being held somewhere thats causing this?

I am able to ssh into the laptop so, this seems to point to some sort of lock
thats not released.

Bisecting radeon drm will be difficult if I have to start from near the
beginning of r600g drm support.