drm/i915: add intel_ddi_connector_get_hw_state

We need this since now on DDI we will have 2 connectors on each
encoder.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>