unifying the vma offset managers

[Dave Airlie]

So a passing comment on irc from Daniel made me look at this, and cleaning
up some surrounding things. This unifies the GEM/TTM vma offset managers
around a single one based on the TTM one.

There is also a patch to cleanup the GEM code after this, and one to clean
up some bits of TTM also.

I've tested it on an intel and a radeon machine and it appears to at least boot.

Dave.