media: vivid: reorder CEC allocation and control set-up

CEC adapters and controllers (handlers) are now set up as follows:

1. Allocate CEC adapters: setup of control handlers in next step
   requires these adapters to be allocated.
2. Setup of control handlers: This must be done prior to registering
   and exposing the adapters to user space to avoid a race condition.
3. Register CEC adapters: make them available to user space.

Signed-off-by: Johan Korsnes <johan.korsnes@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: PTR_ERR -> PTR_ERR_OR_ZERO]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>