linux-dmabuf: Move the attributes part of linux_dmabuf_buffer into its own struct

linux-dmabuf: Move the attributes part of linux_dmabuf_buffer into its own struct

This allows renderers to use that struct to create their own dmabufs,
in case they can’t import the one provided by the client directly but
know how to convert it into a format they can render.

Signed-off-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Differential Revision: https://phabricator.freedesktop.org/D332
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>