drm/encoder: make encoder control functions optional

Simple managed encoders do not require the .destroy callback,
make the whole funcs structure optional.

Signed-off-by: Philipp Zabel <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/drm_encoder.c

···
 	int ret = 0;
 
 	drm_for_each_encoder(encoder, dev) {
-		if (encoder->funcs->late_register)
+		if (encoder->funcs && encoder->funcs->late_register)
 			ret = encoder->funcs->late_register(encoder);
 		if (ret)
 			return ret;
···
 	struct drm_encoder *encoder;
 
 	drm_for_each_encoder(encoder, dev) {
-		if (encoder->funcs->early_unregister)
+		if (encoder->funcs && encoder->funcs->early_unregister)
 			encoder->funcs->early_unregister(encoder);
 	}
 }

drivers/gpu/drm/drm_mode_config.c

···
 			crtc->funcs->reset(crtc);
 
 	drm_for_each_encoder(encoder, dev)
-		if (encoder->funcs->reset)
+		if (encoder->funcs && encoder->funcs->reset)
 			encoder->funcs->reset(encoder);
 
 	drm_connector_list_iter_begin(dev, &conn_iter);

include/drm/drm_encoder.h

···
  * @head: list management
  * @base: base KMS object
  * @name: human readable name, can be overwritten by the driver
- * @funcs: control functions
+ * @funcs: control functions, can be NULL for simple managed encoders
  * @helper_private: mid-layer private data
  *
  * CRTCs drive pixels to encoders, which convert them into signals