[media] exynos4-is: Open shouldn't fail when sensor entity is not linked

[media] exynos4-is: Open shouldn't fail when sensor entity is not linked

In order to allow for automatic media device entities linking
from the level of libv4l plugin the open system call shouldn't
fail, as the libv4l plugins can begin their job not until it
succeeds.
This patch allows for leaving the  pipeline not linked on
open and postpones verifying it to the moment when streamon
callback is called.

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