[media] drivers: media: platform: vivid: Fix possible null derefrence

Check for dev_fmt being null before derefrencing it, to assign it
to planes.

Found using Coccinelle.

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>