[media] exynos4-is: Move common functions to a separate module

Create a common module (exynos4-is-common.ko) for common functions
used across the exynos4-is video device and subdev drivers.

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>