drm/i915/skl: Buffer translation improvements

This patch adds support for 0.85V VccIO on Skylake Y,
separate buffer translation tables for Skylake U,
and support for I_boost for the entries that needs this.

Changes in v2:
* Refactored the code a bit to move all DDI signal level setup to
  intel_ddi.c

Issue: VIZ-5677
Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
[danvet: Apply style polish checkpatch suggested.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>