ARM: exynos_defconfig: enable Exynos DRM Mixer driver

Mixer driver is selected by CONFIG_DRM_EXYNOS_HDMI option. Since Exynos5433
HDMI does not require Mixer. There will be separate options to select Mixer
and HDMI. Adding new option to defconfig before Kconfig will allow to keep
bisectability.

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