[media] media: davinic: vpif_capture: drop started member from struct common_obj

the started member was indicating whether streaming was started
or not, this can be determined by vb2 offering, this patch replaces
the started member from struct common_obj with appropriate vb2 calls.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>