[media] media-device: export the entity function via new ioctl

Now that entities have a main function, expose it via
MEDIA_IOC_G_TOPOLOGY ioctl.

Please notice that some entities may have secundary functions.
Such use case will be addressed later, when we add support for the
Media Controller properties.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>