drm/xen-front: Pass dumb buffer data offset to the backend

While importing a dmabuf it is possible that the data of the buffer
is put with offset which is indicated by the SGT offset.
Respect the offset value and forward it to the backend.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Juergen Gross <jgross@suse.com>