drm/i915/mst: use max link not sink lane count

The source might not support as many lanes as the sink, or the link
training might have failed at higher lane counts. Take these into
account.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/cf59530acafaf9258fb643d321ad251b44f34e29.1491485983.git.jani.nikula@intel.com