[media] exynos4-is: Fix format propagation on FIMC-LITE.n subdevs

FIMC-LITE subdevs have one sink pad and two source pads on which the image
formats are always same. This patch implements missing format propagation
from the sink pad to the source pads, to allow user space to negotiate TRY
format on whole media pipeline involving FIMC-LITE.n subdevs. The subdev
try_fmt helper is simplified.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>