drm/amd/display: Prevent dpcd reads with passive dongles

[why]
During hotplug, a DP port may be connected to the sink through
passive adapter which does not support DPCD reads. Issuing reads
without checking for this condition will result in errors

[how]
Ensure the link is in aux_mode before initiating operation that result
in a DPCD read.

Signed-off-by: Aurabindo Pillai <[e-mail]>
Reviewed-by: Harry Wentland <[e-mail]>
Acked-by: Aurabindo Pillai <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

···
 		dc_sink_retain(aconnector->dc_sink);
 		if (sink->dc_edid.length == 0) {
 			aconnector->edid = NULL;
-			drm_dp_cec_unset_edid(&aconnector->dm_dp_aux.aux);
+			if (aconnector->dc_link->aux_mode) {
+				drm_dp_cec_unset_edid(
+					&aconnector->dm_dp_aux.aux);
+			}
 		} else {
 			aconnector->edid =
-				(struct edid *) sink->dc_edid.raw_edid;
-
+				(struct edid *)sink->dc_edid.raw_edid;
 
 			drm_connector_update_edid_property(connector,
-					aconnector->edid);
-			drm_dp_cec_set_edid(&aconnector->dm_dp_aux.aux,
-					    aconnector->edid);
+							   aconnector->edid);
+
+			if (aconnector->dc_link->aux_mode)
+				drm_dp_cec_set_edid(&aconnector->dm_dp_aux.aux,
+						    aconnector->edid);
 		}
+
 		amdgpu_dm_update_freesync_caps(connector, aconnector->edid);
 		update_connector_ext_caps(aconnector);
 	} else {