drm/i915/dp: debug log max vswing and pre-emphasis

Provide slightly more debugging help.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117150235.22471-1-jani.nikula@intel.com