avcodec/mpeg12dec: always submit the first field to hwaccel

Though this patch to fix ticket #6668, I belive it
is unnecessary to set SLICE_FLAG_ALLOW_FIELD flag to other
hwaccels(dxva, vdpau, etc). Please also refer the orginal comment
of 9cb150c9

Should also fix ticket #8442.
Signed-off-by: Zhong Li <zhong.li@intel.com>
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>

libavcodec/mpeg12dec.c

@@ -1669,8 +1669,7 @@ static int mpeg_field_start(MpegEncContext *s, const uint8_t *buf, int buf_size)
             return AVERROR_INVALIDDATA;
         }
 
-        if (s->avctx->hwaccel &&
-            (s->avctx->slice_flags & SLICE_FLAG_ALLOW_FIELD)) {
+        if (s->avctx->hwaccel) {
             if ((ret = s->avctx->hwaccel->end_frame(s->avctx)) < 0) {
                 av_log(avctx, AV_LOG_ERROR,
                        "hardware accelerator failed to decode first field\n");