lavfi/dnn: used the format name in debug message

Used the format name in debug message.

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavfilter/dnn/dnn_io_proc.c

@@ -70,7 +70,8 @@ DNNReturnType proc_from_dnn_to_frame(AVFrame *frame, DNNData *output, void *log_
         sws_freeContext(sws_ctx);
         return DNN_SUCCESS;
     default:
-        av_log(log_ctx, AV_LOG_ERROR, "do not support frame format %d\n", frame->format);
+        av_log(log_ctx, AV_LOG_ERROR, "do not support frame format %s\n",
+               av_get_pix_fmt_name(frame->format));
         return DNN_ERROR;
     }
 
@@ -127,7 +128,8 @@ DNNReturnType proc_from_frame_to_dnn(AVFrame *frame, DNNData *input, void *log_c
         sws_freeContext(sws_ctx);
         break;
     default:
-        av_log(log_ctx, AV_LOG_ERROR, "do not support frame format %d\n", frame->format);
+        av_log(log_ctx, AV_LOG_ERROR, "do not support frame format %s\n",
+               av_get_pix_fmt_name(frame->format));
         return DNN_ERROR;
     }