[media] exynos4-is: Make fimc-lite independent of the pipeline->subdevs array

Get the sensor subdev by walking media graph in both cases: when the
device is used as a subdev only and through video node. This allows
to not dereference the pipeline->subdevs[] array and makes the module
more generic and easier to re-use in other media driver.

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>