[media] staging: v4l: omap4iss: Use media entity enumeration interface

Instead of using a bitmap directly in a driver, use the new media
entity enumeration interface to perform the same.

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