[media] exynos4-is: Unregister fimc-is subdevs from the media device properly

Add missing v4l2_device_unregister_subdev() call for the FIMC-IS subdevs
(currently there is only the FIMC-IS-ISP subdev) so corresponding resources
are properly freed upon the media device driver module removal.

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