drm/dp/mst: Check peer device type before attempting EDID read

BugLink: http://bugs.launchpad.net/bugs/1642572

commit 4da5caa6 upstream.

Only certain types of pdts have the DDC bus registered, so check for
that before we attempt the EDID read. Othwewise we risk playing around
with an i2c adapter that doesn't actually exist.

Cc: Carlos Santa <carlos.santa@intel.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Tested-by: Carlos Santa <carlos.santa@intel.com>
Tested-by: Kirill A. Shutemov <kirill@shutemov.name>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97666Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1477472755-15288-5-git-send-email-ville.syrjala@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -1160,7 +1160,9 @@ static void drm_dp_add_port(struct drm_dp_mst_branch *mstb,
 			drm_dp_put_port(port);
 			goto out;
 		}
-		if (port->port_num >= DP_MST_LOGICAL_PORT_0) {
+		if ((port->pdt == DP_PEER_DEVICE_DP_LEGACY_CONV ||
+		     port->pdt == DP_PEER_DEVICE_SST_SINK) &&
+		    port->port_num >= DP_MST_LOGICAL_PORT_0) {
 			port->cached_edid = drm_get_edid(port->connector, &port->aux.ddc);
 			drm_mode_connector_set_tile_property(port->connector);
 		}