drm/i915: Check encoder type in enc_to_dig_port()

Don't allow conversion from arbitraty encoder types to a digital port.
Calling enc_to_dig_port() with the wrong encoder may seem far fetched,
but certain paths of the ddi code may be called with hasell's analog
encoder and the conversion is wrong for DP mst encoders too, so safe
guard against it.

v2: Warn if encoder type is unknown and device is not DDI. (Imre)
v3: Remove stray hunk from rebase error. (Ander)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170224141845.5836-1-ander.conselvan.de.oliveira@intel.com

drivers/gpu/drm/i915/intel_drv.h

@@ -1121,7 +1121,19 @@ intel_attached_encoder(struct drm_connector *connector)
 static inline struct intel_digital_port *
 enc_to_dig_port(struct drm_encoder *encoder)
 {
-	return container_of(encoder, struct intel_digital_port, base.base);
+	struct intel_encoder *intel_encoder = to_intel_encoder(encoder);
+
+	switch (intel_encoder->type) {
+	case INTEL_OUTPUT_UNKNOWN:
+		WARN_ON(!HAS_DDI(to_i915(encoder->dev)));
+	case INTEL_OUTPUT_DP:
+	case INTEL_OUTPUT_EDP:
+	case INTEL_OUTPUT_HDMI:
+		return container_of(encoder, struct intel_digital_port,
+				    base.base);
+	default:
+		return NULL;
+	}
 }
 
 static inline struct intel_dp_mst_encoder *