simple-dmabuf-drm: 0 is a valid fd (freedreno)

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>