media: subdev: add streams to v4l2_subdev_get_fmt() helper function

Add streams support to v4l2_subdev_get_fmt() helper function. Subdev
drivers that do not need to do anything special in their get_fmt op can
use this helper directly for v4l2_subdev_pad_ops.get_fmt.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>