drm/i915: Use the same vswing->max_preemph mapping on HSW/BDW as on SKL+

drm/i915: Use the same vswing->max_preemph mapping on HSW/BDW as on SKL+

All DDI platforms support the full set of preemph settings for each
supported vswing, so let's use the same code for them. We'll also move
the code into intel_ddi.c so that it sits closer to the actual buf trans
tables.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517170309.28630-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>