[media] videobuf2-core: call __setup_offsets before buf_init()

Ensure that the offsets are correct before buf_init() is called.
As a consequence the __setup_offsets() function now sets up the
offsets for the given buffer instead of for all new buffers.

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