drm/exynos: dsi: add macros for register access

This patch adds macros for register writing/reading. This is needed for
adding support Exynos5433 MIPI DSI driver, not by using if statement, but
by using driver data.
Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>