drm/exynos: remove the dependency of DP driver for ARCH_EXYNOS

This dependency is a historical thing. It is added when this DP driver is
under media subsystem. Now because it is under Exynos DRM, this dependency
is not needed anymore.

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