drm/dp/mst: Fix off-by-one typo when dump payload table

drm/dp/mst: Fix off-by-one typo when dump payload table

It seems there is a classical off-by-one typo from the beginning
when commit

  ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")

introduced a new helper.

Fix a typo by introducing a macro constant.

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180319141932.37290-1-andriy.shevchenko@linux.intel.com