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>

include/video/samsung_fimd.h

@@ -296,6 +296,7 @@
 
 /* Video buffer addresses */
 #define VIDW_BUF_START(_buff)			(0xA0 + ((_buff) * 8))
+#define VIDW_BUF_START_S(_buff)			(0x40A0 + ((_buff) * 8))
 #define VIDW_BUF_START1(_buff)			(0xA4 + ((_buff) * 8))
 #define VIDW_BUF_END(_buff)			(0xD0 + ((_buff) * 8))
 #define VIDW_BUF_END1(_buff)			(0xD4 + ((_buff) * 8))