lavfi/vf_vpp_qsv: only add the given output sw format to output pad

Otherwise the output format is not changed when output is in system
memory. For example, the output format is still p010le in the following
case:

$ ffmpeg -qsv_device /dev/dri/renderD128 -f lavfi -i testsrc -vf
"format=p010le,vpp_qsv=extra_hw_frames=8:format=nv12" -f null -
...
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf60.4.100
  Stream #0:0: Video: wrapped_avframe, p010le(tv, progressive), 320x240
[SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 25 fps, 25 tbn
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>

libavfilter/vf_vpp_qsv.c

@@ -714,7 +714,8 @@ static const AVOption vpp_options[] = {
 
 static int vpp_query_formats(AVFilterContext *ctx)
 {
-    int ret;
+    VPPContext *vpp = ctx->priv;
+    int ret, i = 0;
     static const enum AVPixelFormat in_pix_fmts[] = {
         AV_PIX_FMT_YUV420P,
         AV_PIX_FMT_NV12,
@@ -727,17 +728,25 @@ static int vpp_query_formats(AVFilterContext *ctx)
         AV_PIX_FMT_QSV,
         AV_PIX_FMT_NONE
     };
-    static const enum AVPixelFormat out_pix_fmts[] = {
-        AV_PIX_FMT_NV12,
-        AV_PIX_FMT_P010,
-        AV_PIX_FMT_QSV,
-        AV_PIX_FMT_NONE
-    };
+    static enum AVPixelFormat out_pix_fmts[4];
 
     ret = ff_formats_ref(ff_make_format_list(in_pix_fmts),
                          &ctx->inputs[0]->outcfg.formats);
     if (ret < 0)
         return ret;
+
+    /* User specifies the output format */
+    if (vpp->out_format == AV_PIX_FMT_NV12 ||
+        vpp->out_format == AV_PIX_FMT_P010)
+        out_pix_fmts[i++] = vpp->out_format;
+    else {
+        out_pix_fmts[i++] = AV_PIX_FMT_NV12;
+        out_pix_fmts[i++] = AV_PIX_FMT_P010;
+    }
+
+    out_pix_fmts[i++] = AV_PIX_FMT_QSV;
+    out_pix_fmts[i++] = AV_PIX_FMT_NONE;
+
     return ff_formats_ref(ff_make_format_list(out_pix_fmts),
                           &ctx->outputs[0]->incfg.formats);
 }