avcodec/frame_thread_encoder: check for frame threading codec cap instead of intra only

It's the correct dedicated capability reported by supported encoders.
Otherwise, the frame thread path will be used for unsupported encoders
like r210 for no gain.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/frame_thread_encoder.c

@@ -120,7 +120,7 @@ int ff_frame_thread_encoder_init(AVCodecContext *avctx, AVDictionary *options){
 
 
     if(   !(avctx->thread_type & FF_THREAD_FRAME)
-       || !(avctx->codec->capabilities & AV_CODEC_CAP_INTRA_ONLY))
+       || !(avctx->codec->capabilities & AV_CODEC_CAP_FRAME_THREADS))
         return 0;
 
     if(   !avctx->thread_count