ffmpeg/qsv: fix QSV-accelerated transcode performance drop issue.

the merge commit 1b04ea1 "avconv: create simple filtergraphs earlier"
will init the filtergraphs earlier, then init the QSV transcode can't
suppose the nb_filters's value, else lead to the QSV transcode performance
drop.

Signed-off-by: Jun Zhao <mypopydev@gmail.com>
Reviewed-by: Ivan Uskov <ivan.uskov@nablet.com
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffmpeg_qsv.c

@@ -210,8 +210,7 @@ int qsv_transcode_init(OutputStream *ost)
 
     /* check if the decoder supports QSV and the output only goes to this stream */
     ist = input_streams[ost->source_index];
-    if (ist->nb_filters || ist->hwaccel_id != HWACCEL_QSV ||
-        !ist->dec || !ist->dec->pix_fmts)
+    if (ist->hwaccel_id != HWACCEL_QSV || !ist->dec || !ist->dec->pix_fmts)
         return 0;
     for (pix_fmt = ist->dec->pix_fmts; *pix_fmt != AV_PIX_FMT_NONE; pix_fmt++)
         if (*pix_fmt == AV_PIX_FMT_QSV)