qsvdec: fix get_format and hwaccel_context handling

This enables the qsv transcoder to actually get activated

libavcodec/qsvdec.c

@@ -58,8 +58,13 @@ static int qsv_decode_init(AVCodecContext *avctx, QSVContext *q, AVPacket *avpkt
                                        AV_PIX_FMT_NV12,
                                        AV_PIX_FMT_NONE };
 
+    ret = ff_get_format(avctx, pix_fmts);
+    if (ret < 0)
+        return ret;
+
+    avctx->pix_fmt      = ret;
+
     q->iopattern  = MFX_IOPATTERN_OUT_SYSTEM_MEMORY;
-    if (!q->session) {
     if (avctx->hwaccel_context) {
         AVQSVContext *qsv = avctx->hwaccel_context;
 
@@ -69,14 +74,15 @@ static int qsv_decode_init(AVCodecContext *avctx, QSVContext *q, AVPacket *avpkt
         q->nb_ext_buffers = qsv->nb_ext_buffers;
     }
     if (!q->session) {
+        if (!q->internal_qs.session) {
             ret = ff_qsv_init_internal_session(avctx, &q->internal_qs,
                                                q->load_plugins);
             if (ret < 0)
                 return ret;
+        }
 
         q->session = q->internal_qs.session;
     }
-    }
 
     if (avpkt->size) {
         bs.Data       = avpkt->data;
@@ -123,11 +129,6 @@ static int qsv_decode_init(AVCodecContext *avctx, QSVContext *q, AVPacket *avpkt
         return ff_qsv_error(ret);
     }
 
-    ret = ff_get_format(avctx, pix_fmts);
-    if (ret < 0)
-        return ret;
-
-    avctx->pix_fmt      = ret;
     avctx->profile      = param.mfx.CodecProfile;
     avctx->level        = param.mfx.CodecLevel;
     avctx->coded_width  = param.mfx.FrameInfo.Width;