cenalloc: Constraint-Enabled Allocation helpers for dma-buf

Devices sharing buffers using dma-buf could benefit from sharing their
constraints via struct device, and dma-buf framework would manage the
common constraints for all attached devices per buffer.

With that information, we could have a 'generic' allocator helper in
the form of a central dma-buf exporter, which can create dma-bufs, and
allocate backing storage at the time of first call to
dma_buf_map_attachment.

This allocation would utilise the constraint-mask by matching it to
the right allocator from a pool of allocators, and then allocating
buffer backing storage from this allocator.

The pool of allocators could be platform-dependent, allowing for
platforms to hide the specifics of these allocators from the devices
that access the dma-buf buffers.

A sample sequence could be:
- get handle to cenalloc_device,
- create a dmabuf using cenalloc_buffer_create;
- use this dmabuf to attach each device, which has its constraints
   set in the constraints mask (dev->dma_params->access_constraints_mask)
  - at each dma_buf_attach() call, dma-buf will check to see if the constraint
    mask for the device requesting attachment is compatible with the constraints
    of devices already attached to the dma-buf; returns an error if it isn't.
- after all devices have attached, the first call to dma_buf_map_attachment()
  will allocate the backing storage for the buffer.
- follow the dma-buf api for map / unmap etc usage.
- detach all attachments,
- call cenalloc_buffer_free to free the buffer if refcount reaches zero;

** IMPORTANT**
This mechanism of delayed allocation based on constraint-enablement will work
*ONLY IF* the first map_attachment() call is made AFTER all attach() calls are
done.

Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org

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