drm/exynos: add macro to get the address of START_S reg

This macro is need to get the value of the START shadow register, that
will tell if an framebuffer is currently displayed on the screen or not.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>