hwcontext_vaapi: remove duplicate formats from sw_format list

hwcontext_vaapi maps different VA fourcc to the same pix_fmt for U/V
plane swap cases, however duplicate formats are not expected in sw_format
list when merging formats.

For example:
ffmpeg -loglevel debug -init_hw_device vaapi -filter_hw_device vaapi0 \
-f lavfi -i smptebars -vf \
"hwupload=derive_device=vaapi,scale_vaapi,hwdownload,format=yuv420p" \
-vframes 1 -f null -

Without this fix, an auto scaler is required for the above command
   Duplicate formats in ff_merge_formats detected
   [auto_scaler_0 @ 0x560df58f4550] Setting 'flags' to value 'bicubic'
   [auto_scaler_0 @ 0x560df58f4550] w:iw h:ih flags:'bicubic' interl:0
   [Parsed_hwupload_0 @ 0x560df58f0ec0] auto-inserting filter
   'auto_scaler_0' between the filter 'graph 0 input from stream 0:0' and
   the filter 'Parsed_hwupload_0'

Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>

libavutil/hwcontext_vaapi.c

@@ -268,14 +268,24 @@ static int vaapi_frames_get_constraints(AVHWDeviceContext *hwdev,
             }
 
             for (i = j = 0; i < attr_count; i++) {
+                int k;
+
                 if (attr_list[i].type != VASurfaceAttribPixelFormat)
                     continue;
                 fourcc = attr_list[i].value.value.i;
                 pix_fmt = vaapi_pix_fmt_from_fourcc(fourcc);
-                if (pix_fmt != AV_PIX_FMT_NONE)
+
+                if (pix_fmt == AV_PIX_FMT_NONE)
+                    continue;
+
+                for (k = 0; k < j; k++) {
+                    if (constraints->valid_sw_formats[k] == pix_fmt)
+                        break;
+                }
+
+                if (k == j)
                     constraints->valid_sw_formats[j++] = pix_fmt;
             }
-            av_assert0(j == pix_fmt_count);
             constraints->valid_sw_formats[j] = AV_PIX_FMT_NONE;
         }
     } else {
@@ -287,9 +297,19 @@ static int vaapi_frames_get_constraints(AVHWDeviceContext *hwdev,
             err = AVERROR(ENOMEM);
             goto fail;
         }
-        for (i = 0; i < ctx->nb_formats; i++)
-            constraints->valid_sw_formats[i] = ctx->formats[i].pix_fmt;
-        constraints->valid_sw_formats[i] = AV_PIX_FMT_NONE;
+        for (i = j = 0; i < ctx->nb_formats; i++) {
+            int k;
+
+            for (k = 0; k < j; k++) {
+                if (constraints->valid_sw_formats[k] == ctx->formats[i].pix_fmt)
+                    break;
+            }
+
+            if (k == j)
+                constraints->valid_sw_formats[j++] = ctx->formats[i].pix_fmt;
+        }
+
+        constraints->valid_sw_formats[j] = AV_PIX_FMT_NONE;
     }
 
     constraints->valid_hw_formats = av_malloc_array(2, sizeof(pix_fmt));