[PATCH] xfree86: detach scanout pixmaps when detaching output GPUs

[Dave Airlie]

> Our internal QA and end users are continuing to report this crash.
> E.g.,
> https://devtalk.nvidia.com/default/topic/551709/linux/319-32-the-infamous-blank-black-screen-/post/3859909/#3859909

I've totally never managed to make it happen and it still seems like
it should obviously happen everwhere,

but if it fixes things,

Reviewed-by: Dave Airlie <airlied at redhat.com>

Dave.