drm/exynos: Select VIDEOMODE_HELPERS for FIMD

FIMD also requires video mode helper APIs.
Without this patch we get the following build error:
drivers/gpu/drm/exynos/exynos_drm_fimd.c:895: undefined reference to
`of_get_fb_videomode'
make: *** [vmlinux] Error 1

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>