convert dc to using krefs for object reference counts

[Dave Airlie]

This series might be a bit of a too big step :-)

So in the kernel we use krefs for reference counts for lots of good
reasons, the main one, is no bugs with roll your own atomic reference
counting for structs, which lots of this code is.

I've ported all the structs I could find using hand rolled atomic
reference counters to use krefs.

Dave.