[media] media: i.MX27 camera: more efficient discard buffer handling

Some elements of 'mx2_buffer' are grouped together in another
auxiliary structure. This way we don't need to have unused
'vb2_buffer' structures for both discard buffers.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>