drm prime/dma-buf import lifetime issue

[Dave Airlie]

>
> b) don't keep track of dma_buf, keep track of gem objects, when we get
> a lookup, check inside the gem object, since currently we NULL out the
> export_dma_buf when we hit the release path, apart from the fact I'm
> sure the locking is foobar,

Okay I've sent and implementation of this option, however it does mean
that prime relies on GEM, which at this point is a tradeoff I'm quite
happy to start making.

Dave.