drm/i915/skl: Enable DDI-E

There are OEMs using DDI-E out there,
so let's enable it.

Unfortunately there is no detection bit for DDI-E
So we need to rely on VBT for that.

I also need to give credits to Xiong since before seing
his approach to check info->support_* I was creating an ugly
vbt->ddie_sfuse_strap in order to propagate the ddi presence info

v2: Rebased as last patch in the series. since all other patches
in this series are needed for anything working propperly on DDI-E.

Credits-to: "Zhang, Xiong Y" <xiong.y.zhang@intel.com>
Cc: "Zhang, Xiong Y" <xiong.y.zhang@intel.com>
Reviewed-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>