Convert pxa_camera from videobuf to videobuf2.

As the soc_camera was already compatible with videobuf2, the port is
quite straightforward.

The special case of this code in which the vb2 to prepare is "too
big" in terms of size for the new capture format, the pxa_camera will
fail.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>