[rfc] drm sync objects (search for spock)

[Dave Airlie]

Okay I've gone around the sun with these a few times, and
pretty much implemented what I said last week.

This is pretty much a complete revamp.

1. sync objects are self contained drm objects, they
have a file reference so can be passed between processes.

2. Added a sync object wait interface modelled on the vulkan
fence waiting API.

3. sync_file interaction is explicitly different than
opaque fd passing, you import a sync file state into an
existing syncobj, or create a new sync_file from an
existing syncobj. This means no touching the sync file code
at all. \o/

I haven't used rcu anywhere here, I've used xchg to swap
fence pointers in the hope that's safe. If this does need
rcu'ing I suggest we do it in a follow on patch to minimise
the review pain.

Dave.