[media] v4l: add g_dv_preset callback to V4L2 subdev

Callback is used to acquire current digital video preset from a subdev.
It is used to avoid keeping dv preset in top-level driver.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>