media: exynos4-is: Check pipe is valid before calling subdev

If the subdev is not yet present (probably because the subdev
module has not yet been loaded), the pipe will be NULL. Make sure
that this is not the case before attempting to call the op.

Signed-off-by: Simon Shields <simon@lineageos.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>