drm/i915/dp/mst: Add source port info to debugfs output

Modify the debugfs output for i915_dp_mst_info to list the source port for
the DP MST topology in question.

v2: rebase
v3: rebase
v4: rebase

cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NJim Bride <jim.bride@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460654317-31288-3-git-send-email-jim.bride@linux.intel.com

drivers/gpu/drm/i915/i915_debugfs.c

@@ -3434,7 +3434,8 @@ static int i915_dp_mst_info(struct seq_file *m, void *unused)
 		intel_dig_port = enc_to_dig_port(encoder);
 		if (!intel_dig_port->dp.can_mst)
 			continue;
-
+		seq_printf(m, "MST Source Port %c\n",
+			   port_name(intel_dig_port->port));
 		drm_dp_mst_dump_topology(m, &intel_dig_port->dp.mst_mgr);
 	}
 	drm_modeset_unlock_all(dev);