drm/dp: Add drm_dp_channel_coding_supported() helper

Add a helper to check whether the sink supports ANSI 8B/10B channel
coding capability as specified in ANSI X3.230-1994, clause 11.
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021143437.1477719-5-thierry.reding@gmail.com

include/drm/drm_dp_helper.h

@@ -137,6 +137,7 @@
 # define DP_DETAILED_CAP_INFO_AVAILABLE	    (1 << 4) /* DPI */
 
 #define DP_MAIN_LINK_CHANNEL_CODING         0x006
+# define DP_CAP_ANSI_8B10B		    (1 << 0)
 
 #define DP_DOWN_STREAM_PORT_COUNT	    0x007
 # define DP_PORT_COUNT_MASK		    0x0f
@@ -1287,6 +1288,12 @@ drm_dp_sink_supports_fec(const u8 fec_capable)
 	return fec_capable & DP_FEC_CAPABLE;
 }
 
+static inline bool
+drm_dp_channel_coding_supported(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
+{
+	return dpcd[DP_MAIN_LINK_CHANNEL_CODING] & DP_CAP_ANSI_8B10B;
+}
+
 /*
  * DisplayPort AUX channel
  */