ARM: multi_v7_defconfig: Enable Exynos DRM Mixer driver

Mixer driver is selected by CONFIG_DRM_EXYNOS_HDMI option. Since Exynos5433
HDMI does not require Mixer and there are separate options to select Mixer
and HDMI.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>