drm/dp: add MSO related DPCD registers

Add DPCD register definitions for eDP 1.4 Multi-SST Operation.

Cc: Nischal Varide <nischal.varide@intel.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: NUma Shankar <uma.shankar@intel.com>
Acked-by: NDaniel Vetter <daniel@ffwll.ch>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ab57627f373ec4a80494bb51ea51080810d9bfb0.1613054234.git.jani.nikula@intel.com

include/drm/drm_dp_helper.h

@@ -1016,6 +1016,11 @@ struct drm_device;
 #define DP_EDP_REGIONAL_BACKLIGHT_BASE      0x740    /* eDP 1.4 */
 #define DP_EDP_REGIONAL_BACKLIGHT_0	    0x741    /* eDP 1.4 */
 
+#define DP_EDP_MSO_LINK_CAPABILITIES        0x7a4    /* eDP 1.4 */
+# define DP_EDP_MSO_NUMBER_OF_LINKS_MASK    (7 << 0)
+# define DP_EDP_MSO_NUMBER_OF_LINKS_SHIFT   0
+# define DP_EDP_MSO_INDEPENDENT_LINK_BIT    (1 << 3)
+
 /* Sideband MSG Buffers */
 #define DP_SIDEBAND_MSG_DOWN_REQ_BASE	    0x1000   /* 1.2 MST */
 #define DP_SIDEBAND_MSG_UP_REP_BASE	    0x1200   /* 1.2 MST */