[media] marvell-cam: convert to videobuf2

This is a basic, naive conversion to the videobuf2 infrastructure, removing
a lot of code in the process.  For now, we're using vmalloc, which is
suboptimal, but it does match what the cafe driver did before.  In the cafe
case, it may have to stay that way just because memory is too tight to do
direct streaming; mmp-camera will be able to do better.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>