[media] s5p-fimc: Change platform subdevs registration method

The previous method of registering platform entities into the main
driver using driver_find() and then iterating over devices bound to
a driver was racy and is being removed here. Nothing was preventing
module from unloading during a call to try_module_get(driver->owner).
Instead, we look up a device first and then check for its driver while
holding device lock.
The platform sub-devices are looked up and registered to the top
level driver. When any sub-device is not yet initialized and ready
the main driver's probe() will be deferred.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>