[media] media: videobuf2-core.h: add a helper to get status of start_streaming()

this patch adds a helper to get the status if start_streaming()
was called successfully.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Cc: Pawel Osciak <pawel@osciak.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>