[media] exynos4-is: Fix sensor subdev -> FIMC notification setup

Ensure the v4l2_device notifications from sensor subdev works
also after the media links reconfiguration.

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>