drm/exynos: Add dependency for G2D in Kconfig

Select Exynos DRM based G2D only if non-DRM based Exynos G2D driver
is not selected.

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