drm/exynos: Add the dependency for DRM_EXYNOS to DPI/DSI/DP

Without this dependency, Kbuild is confused and the configs below
them are not placed under Exynos DRM. This patch fixes it, so the
configs below them become to be placed under Exynos DRM.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>