ARM: exynos_defconfig: Enable vivid driver as a module

The Virtual Video Test Driver can be used to emulate video capture and
output devices so it's very useful for testing. Since is not necessary
to boot, can be enabled as module to avoid increasing the kernel size.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>