media: sun6i-csi: Move csi buffer definition to main header file

The buffer structure is a top-level definition, put it in the main header
to keep things tidy. No functional change intended.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>