[repost] drm sync objects cleaned up

[Dave Airlie]

This set of sync object patches should address most of the issues
raised in review.

The major changes:
Race on destroy should be gone,
Documentation fixups
amdgpu internal use p instead of adev fixups

My plans are to write some igt tests this week, and try
to get some more review on what the API should allow (should
I lock it down to drm syncobj is just semaphores for now).

Dave.