drm/i915/mst: Print active mst links after update

Both mst_disable_dp and mst_post_disable_dp print number of active links
before the variable has been updated. Move the print statement in
mst_disable_dp after the decrement so that the printed values indicate
the disabing of a mst connector. Also, add some text to clarify what we
are printing.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170918222141.4674-2-dhinakaran.pandiyan@intel.com