avcodec/libsvtav1: update avctx bit rate according to RC mode

This way we can filter out the default value for this member, which
is nonzero. Bases on the current affairs that bit rate based rate
control is nonzero in SVT-AV1.

libavcodec/libsvtav1.c

@@ -297,7 +297,8 @@ static int config_enc_params(EbSvtAv1EncConfiguration *param,
         param->profile = FF_PROFILE_AV1_HIGH;
     }
 
-    avctx->bit_rate                 = param->target_bit_rate;
+    avctx->bit_rate = param->rate_control_mode > 0 ?
+                      param->target_bit_rate : 0;
 
     return 0;
 }