[media] marvell-cam: core code reorganization

This code shows signs of having been mucked with over the last five years
or so; things were kind of mixed up.  This patch reorders functions into a
more rational organization which, with luck, will facilitate making the
buffer modes selectable at configuration time.  Code movement only: no
functional changes here.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>