[media] mx2_camera: Remove buffer states

After removing i.mx25 support and buf_cleanup() callback,
buffer states are not used in the code any longer.

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>