[media] vivid: default to single planar device instances

The default used to be that the first vivid device instance was
single planar, the second multi planar, the third single planar, etc.

However, that turned out to be unexpected and awkward. Change the
driver to always default to single planar.

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