simple-dmabuf-v4l: Add support to NV12 devices that combine planes

There are V4L2 devices that will output NV12 but will do so using one dma
buffer. To support this, we need to add the same dma buffer twice but with
a different offset for the chrominance plane.
Also supports situations of 3 planes (e.g. YU12) inside a single dma buffer.

Fixes: #712
Signed-off-by: Bram Stolk <(b.stolk@gmail.com)>