drm/dp: Drop open-coded drm_dp_is_branch() in drm_dp_read_downstream_info()

Noticed this while fixing another issue in drm_dp_read_downstream_info(),
the open coded DP_DOWNSTREAMPORT_PRESENT check here just duplicates what we
already do in drm_dp_is_branch(), so just get rid of it.
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430223428.10514-2-lyude@redhat.com

drivers/gpu/drm/drm_dp_helper.c

@@ -677,9 +677,7 @@ int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
 	memset(downstream_ports, 0, DP_MAX_DOWNSTREAM_PORTS);
 
 	/* No downstream info to read */
-	if (!drm_dp_is_branch(dpcd) ||
-	    dpcd[DP_DPCD_REV] < DP_DPCD_REV_10 ||
-	    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
+	if (!drm_dp_is_branch(dpcd) || dpcd[DP_DPCD_REV] < DP_DPCD_REV_10)
 		return 0;
 
 	/* Some branches advertise having 0 downstream ports, despite also advertising they have a