lavc/vaapi_encode: Don't return error if the underlying driver doesn't support B frames

The underlying driver need not support B frames - since they are enabled
by default for some codecs, it is better to disable them rather than
returning an error in this case.  This makes the default settings usable
for low-power encoding on Intel platforms.
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>

libavcodec/vaapi_encode.c

@@ -1096,10 +1096,10 @@ static av_cold int vaapi_encode_config_attributes(AVCodecContext *avctx)
                 goto fail;
             }
             if (avctx->max_b_frames > 0 && ref_l1 < 1) {
-                av_log(avctx, AV_LOG_ERROR, "B frames are not "
-                       "supported (%#x).\n", attr[i].value);
-                err = AVERROR(EINVAL);
-                goto fail;
+                av_log(avctx, AV_LOG_WARNING, "B frames are not "
+                       "supported (%#x) by the underlying driver.\n",
+                       attr[i].value);
+                avctx->max_b_frames = 0;
             }
         }
         break;