Try to address the DMA-buf coherency problem

[Christian König]

Hi guys,

after finding that we essentially have two separate worlds for coherent sharing
of buffer through DMA-buf I thought I will tackle that problem a bit and at
least allow the framework to reject attachments which won't work.

So those patches here add a new dma_coherent flag to each DMA-buf object
telling the framework that dev_is_dma_coherent() needs to return true for an
importing device to be able to attach. Since we should always have a fallback
path this should give userspace the chance to still keep the use case working,
either by doing a CPU copy instead or reversing the roles of exporter and
importer.

For DRM and most V4L2 devices I then fill in the dma_coherent flag based on the
return value of dev_is_dma_coherent(). Exporting drivers are allowed to clear
the flag for their buffers if special handling like the USWC flag in amdgpu or
the uncached allocations for radeon/nouveau are in use.

Additional to that importers can also check the flag if they have some
non-snooping operations like the special scanout case for amdgpu for example.

The patches are only smoke tested and the solution isn't ideal, but as far as
I can see should at least keep things working.

Please review and/or comment,
Christian.