[media] media: davinci: vpif_display: improve start/stop_streaming callbacks

this patch drops unnecessary check from start_streaming() callback
as this is already done in try/s_fmt and some minor code cleanups,
drops check for vb2_is_streaming() as this check is done by vb2
itself before calling this callback.

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>