drm/i915/dp: constify link_status

Follow-up to
commit 0aec2881
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Fri Sep 27 19:01:01 2013 +0300

    drm/dp: constify DP DPCD helpers

Requested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>