drm/exynos: dp: Use DPCD defines of drm_dp_helper.h

Use DPCD defines of drm_dp_helper.h; thus, duplicated DPCD defines
of exynos_dp_core.h can be removed. Also, DP_TEST_EDID_CHECKSUM
define is added to drm_dp_helper.h. There is no functional change.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>