drm/exynos: enable OF_VIDEOMODE and FB_MODE_HELPERS for exynos drm fimd

patch adds "select OF_VIDEOMODE" and "select FB_MODE_HELPERS" when
EXYNOS_DRM_FIMD config is selected. Also adds the "OF" dependency.

Signed-off-by: Vikas Sajjan <vikas.sajjan@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>