drm/dp/mst: Enhance DP MST debugfs output

Add some additional information (input vs. output port, sink associated
with VC, peer device type, max number of VCs supported) and ensure that
any embedded '\0' characters in a branch device's devid string are not
written to debugfs.

v2: Rebase + change drm_edid_get_monitor_name() call to reflect new
    signature.

v3: Minor changes suggested by Jani + rebase.

v4: Rebase

cc: dri-devel@lists.freedesktop.org
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/1460654317-31288-2-git-send-email-jim.bride@linux.intel.com