drm/i915: Introduce intel_encoder_is_dig_port()

Add intel_encoder_is_dig_port() to match intel_encoder_is_dp().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705164357.28512-3-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>