drm/i915/skl: Enable DDI-E

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" <[email protected]>
Cc: "Zhang, Xiong Y" <[email protected]>
Reviewed-by: Xiong Zhang <[email protected]>
Signed-off-by: Rodrigo Vivi <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>