[media] vivid-tpg: separate planes and buffers

Add a new field that contains the number of buffers. This may be
less than the number of planes in case multiple planes are combined
into one buffer.

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