media: via-camera: convert to the vb2 framework

Convert the via-camera to the vb2 framework. With this change this
driver passes all 'v4l2-compliance -s' tests on my OLPC 1.5.

Also tested with the Sugar 'Record' application.

All tests were done under the OLPC official 5.0.8 kernel.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>