drm/i915/skl: Add support for DP voltage swings and pre-emphasis

They are similar to Haswell.

v2: Rebased on top of drm-intel-nightly
v3: Rebased on top of Sonika's DP train defines renaming

Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>