[media] exynos4-is: Correct clean up sequence on error path in fimc_is_probe()

The memory allocator is being initialized before registering the subdevs
so reverse the cleanup sequence to avoid trying unregister not registered
subdevs.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>