[media] s5p-fimc: Fix setup of initial links to FIMC entities

This patch fixes regression introduced in commit
4af81310. Missing pointers
to the sensor subdevs are restored along with subdev callback
notifications to a corresponding FIMC instance driver.

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