[media] videobuf2-core: move __setup_lengths into __vb2_queue_alloc()

Rather than setting up the lengths at the end, set them up when
the vb2_buffer is allocated. This also ensures that buf_init()
sees the right length values.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>