[media] vb2: allow read/write as long as the format is single planar

It was impossible to read() or write() a frame if the queue type was multiplanar.
Even if the current format is single planar. Change this to just check whether
the number of planes is 1 or more.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>