drm/i915: Integrate BXT into intel_ddi_dp_voltage_max()

Make BXT less special by following the CNL approach and handling
it in intel_ddi_dp_voltage_max() alognside every other DDI platform.

v2: Clean up the argument types to bxt_ddi_vswing_sequence() while at it

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171016145705.11780-6-ville.syrjala@linux.intel.com


Reviewed-by: James Ausmus <james.ausmus@intel.com>