[media] exynos4-is: Refactor vidioc_s_fmt, vidioc_try_fmt handlers

Remove duplicated code in the vidioc_try_fmt and vidioc_s_fmt handlers.
This is a pre-requsite to allow successful fimc.capture video open even
if its corresponding media entities are not linked into a complete
pipeline.

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>