[media] staging: v4l: omap4iss: Use the new media graph walk interface

The media graph walk requires initialisation and cleanup soon. Update the
users to perform the soon necessary API calls.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>