drm/dp: Dump downstream facing port caps

It helps when the logs have a dump of the DFP capabilities.

v2: Move the dumping to the new helper
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200904115354.25336-2-ville.syrjala@linux.intel.comReviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_dp_helper.c

@@ -545,8 +545,13 @@ int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
 	ret = drm_dp_dpcd_read(aux, DP_DOWNSTREAM_PORT_0, downstream_ports, len);
 	if (ret < 0)
 		return ret;
+	if (ret != len)
+		return -EIO;
+
+	DRM_DEBUG_KMS("%s: DPCD DFP: %*ph\n",
+		      aux->name, len, downstream_ports);
 
-	return ret == len ? 0 : -EIO;
+	return 0;
 }
 EXPORT_SYMBOL(drm_dp_read_downstream_info);