media: subdev: add v4l2_subdev_get_fmt() helper function

Add v4l2_subdev_get_fmt() helper function which implements
v4l2_subdev_pad_ops.get_fmt using active state. Subdev drivers that
support active state and 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>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>