[media] media: davinci: vpif_capture: drop buf_init() callback

this patch drops the buf_init() callback as init
of buf list is not required.

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>