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