drm/i915: add port field to intel_digital_port

Both "intel_dp" and "intel_hdmi" structs had a "port" field, which
always had the same value. It makes more sense to move this to
intel_digital_port, so we can know the port independently of the
connector type.

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>