drm/i915/bdw: add support for BDW DP voltage swings and pre-emphasis

They're not the same as the Haswell ones.

Reviewed-by: Art Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>