[media] vb2-core: refactor reqbufs/create_bufs

Split off the memory and type validation. This is done both from reqbufs
and create_bufs, and will also be done by vb2 helpers in a later patch.

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