drm/exynos: depend on ARCH_EXYNOS for DRM_EXYNOS

Because PLAT_SAMSUNG isn't include exynos SoCs for arm64, but
ARCH_EXYNOS can do it. And it also needs to add ARCH_S3C64XX instead of
PLAT_SAMSUNG.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>