drm/i915/ehl: Enable DDI-D

EHL has four DDI's (DDI-A and DDI-D share combo PHY A).

Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190709183934.445-6-matthew.d.roper@intel.com