drm/exynos: fixed build dependency for DRM_EXYNOS_FIMD

FB based FIMD and DRM based FIMD drivers use same hardware
so with this patch, only one of them would be selected.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>