Bunch of DMA-buf cleanup patches

[Christian König]

Hello everyone,

just a few DMA-buf cleanup patches. The first one is fixing an incorrect
documentation which has annoyed me for quite a while.

The rest basically just removes stuff we no longer need or which was
just added as abstraction which was never used.

Please review and/or voice objection if some of that is till needed for
something.

Regards,
Christian.