drm/i915: Set power domain for DDI-E

DDI-E and DDI-A share 4 the same DDI-A lanes.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>