[media] videobuf2-core: fill in length field for multiplanar buffers

length should be set to num_planes in __fill_v4l2_buffer(). That way the
caller knows how many planes there are in the buffer.

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