drm/exynos: Fix build error with exynos_drm_connector.c

exynos_drm_connector.c now uses videomode helper API. Hence select
VIDEOMODE_HELPERS at DRM_EXYNOS level itself instead of at
DRM_EXYNOS_FIMD to avoid the following compilation error when FIMD
is not selected (introduced by commit cf796235a6 "drm/exynos: fimd:
replace struct fb_videomode with videomode"):
drivers/built-in.o: In function `exynos_drm_connector_get_modes':
drivers/gpu/drm/exynos/exynos_drm_connector.c:86:
undefined reference to `drm_display_mode_from_videomode'

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