drm/i915: Use intel_ddi_dp_voltage_max() for HSW/BDW too

Use intel_ddi_dp_voltage_max() for HSW/BDW too instead of letting these
fall through the if ladder in a weird way. This function will look at
the actual buf trans tables we have for HSW/BDW to determine the max
vswing level.

It looks to me like the current code leads HSW port A down the IVB port
A path, HSW port B+ and BDW fall through to the very end. Both cases do
result in the correct max vswing level 2, but it's very hard to see that
from the code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517170309.28630-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>