[media] videobuf2-core: remove duplicated code

Remove duplicated test of buffer presence at streamon

Signed-off-by: Victor Lambret <victor.lambret.ext@parrot.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>