[media] exynos4-is: make VIDEO_SAMSUNG_EXYNOS4_IS tristate

With CONFIG_V4L2=m and VIDEO_SAMSUNG_EXYNOS4_IS=y, we can select the
individual drivers as built-in code when that should not be possible:

drivers/built-in.o: In function `s5pcsis_set_fmt':
policy.c:(.text+0x13afdc): undefined reference to `v4l_bound_align_image'
drivers/built-in.o: In function `s5pcsis_probe':
policy.c:(.text+0x13b440): undefined reference to `v4l2_of_parse_endpoint'
policy.c:(.text+0x13b72c): undefined reference to `v4l2_subdev_init'

Changing VIDEO_SAMSUNG_EXYNOS4_IS to tristate means that the dependency
from CONFIG_V4L2 propates to the individual Kconfig symbols and they
can only be built as loadable modules if V4L2 or any other of the
dependencies itself is a module.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>