drm/i915: Enable HDMI on DDI-E

DDI-E doesn't have the correspondent GMBUS pin.

We rely on VBT to tell us which one it being used instead.

The DVI/HDMI on shared port couldn't exist.

This patch isn't tested without hardware wchich has HDMI
on DDI-E.

v2: fix trailing whitespace
v3: MISSING_CASE take place of BUG()

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