Dave Airlie pointed out on IRC that idr_replace lets us know if the ID hasn't
been allocated, so we don't need a special pointer value for allocated IDs that
don't have valid information yet.
There's a difference between a drawable ID not having valid drawable
information and not being allocated at all. Not making the distinction would
break i915 DRM swap scheduling with older X servers that don't push drawable
cliprect information to the DRM.
The current version didn't build on BSD, where the new functionality isn't used
yet anyway. Whoever changes that will hopefully be able to make the OSes share
this file as well.
Actually make the existing ioctls for adding and removing drawables do
something useful, and add another ioctl for the X server to update drawable
information. The only kind of drawable information tracked so far is cliprects.
