drm/i915: Implement ibx_digital_port_connected() for IBX
CPT+ PCHs have different bit definition to read the HPD live status. I
don't have an ILK with digital ports handy, which is why this patch is
separate from the CPT+ implementation. If the docs don't lie, it should
all be fine though.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> Reviewed-by: Jani Nikula <jani.nikula@intel.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>