[media] videobuf2-core: fill in q->bufs[vb->index] before buf_init()

Fill in q->bufs[vb->index] before the call to buf_init: it makes
sense that this is initialized correctly.

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