[media] cec: pass parent device in register(), not allocate()

The cec_allocate_adapter function doesn't need the parent device, only the
cec_register_adapter function needs it.

Drop the cec_devnode parent field, since devnode.dev.parent can be used
instead.

This change makes the framework consistent with other frameworks where the
parent device is not used until the device is registered.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>