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.comReviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/intel_drv.h

@@ -1245,23 +1245,29 @@ intel_attached_encoder(struct drm_connector *connector)
 	return to_intel_connector(connector)->encoder;
 }
 
-static inline struct intel_digital_port *
-enc_to_dig_port(struct drm_encoder *encoder)
+static inline bool intel_encoder_is_dig_port(struct intel_encoder *encoder)
 {
-	struct intel_encoder *intel_encoder = to_intel_encoder(encoder);
-
-	switch (intel_encoder->type) {
+	switch (encoder->type) {
 	case INTEL_OUTPUT_DDI:
-		WARN_ON(!HAS_DDI(to_i915(encoder->dev)));
-		/* fall through */
 	case INTEL_OUTPUT_DP:
 	case INTEL_OUTPUT_EDP:
 	case INTEL_OUTPUT_HDMI:
+		return true;
+	default:
+		return false;
+	}
+}
+
+static inline struct intel_digital_port *
+enc_to_dig_port(struct drm_encoder *encoder)
+{
+	struct intel_encoder *intel_encoder = to_intel_encoder(encoder);
+
+	if (intel_encoder_is_dig_port(intel_encoder))
 		return container_of(encoder, struct intel_digital_port,
 				    base.base);
-	default:
+	else
 		return NULL;
-	}
 }
 
 static inline struct intel_dp_mst_encoder *