staging/android: Update ION TODO per LPC discussion

We discussed a bit with the folks on the Cc: list below what to do
with ION. Two big take-aways:

- High-performance drivers (like gpus) always want to play tricks with
  coherency and will lie to the dma api (radeon, nouveau, i915 gpu
  drivers all do so in upstream). What needs to be done here is fill
  gaps in dma-buf so that we can do this without breaking the dma-api
  expections of other clients like v4l. The consesus is that hw won't
  stop needing these tricks anytime soon.

- Placement constraints for shared buffers won't be solved any other
  way than through something platform-specific like ion with
  platform-specific knowledge in userspace in something like gralloc.
  For general-purpose devices where this assumption would be painful
  for userspace (like servers) the consensus is that such devices will
  have proper MMUs where placement constraint handling is fairly
  irrelevant.

Hence it is reasonable to destage ion as-is without changing the
overall design to enable these use-cases and just fixing up a these
few fairly minor things. Since there won't relly be an open-source
userspace for ion (and hence drm maintainers won't take it) the
proposal is to eventually move it to drivers/android/ion.[hc]. Laura
would be ok with being maintainer once this is all done and ion is
destaged.

Note that Thiago is working on exposing the cpu cache flushing for
cpu access from userspace through mmaps so this is alread in progress.
Also adding him to the Cc: list.

v2: Add ION_IOC_IMPORT to the list of ioctl that probably should go.

Cc: Laura Abbott <labbott@redhat.com>
Cc: sumit.semwal@linaro.org
Cc: laurent.pinchart@ideasonboard.com
Cc: ghackmann@google.com
Cc: robdclark@gmail.com
Cc: david.brown@arm.com
Cc: romlem@google.com
Cc: Tiago Vignatti <tiago.vignatti@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>