Refcounting dma_resv and using that for drm_exec support in TTM

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Hi everybody,

The idea of ref-counting dma_resv or ww_mutex came up multiple times from
different people, but so far at least I have abandoned that as to
complicated to implement considering how widely used that object is.

Thanks to AI I gave the task to refcount dma_resv to Claude Sonet 4 just
to check how horrible it would look like.

Well turns out that this is actually a cleanup we should most likely aim
for and I'm really wondering why we haven't done it like this in the
first place.

Not only resolves it a bunch of issues with dma_resv instances shared by
multiple GEM objects (we just recently had a bunch of patches for that on
the mailing list), but also allows TTM to implement it's delayed delete
handling without any zombie resurrection or similar hacks.

The patch set is totally work in progress and only survives a smoke test
with amdgpu, but I still wanted to check if the idea is valuable and
should be looked in further.

Please comment and/or tear apart :)

Cheers,
Christian.