dxva2_h264: fix signaling of mbaff frames

The MBAFF flag may only be signaled if we're actually dealing with
a full frame, and not singular fields, as it can happen in mixed content.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/dxva2_h264.c

@@ -93,7 +93,8 @@ static void fill_picture_parameters(struct dxva_context *ctx, const H264Context
     pp->num_ref_frames                = h->sps.ref_frame_count;
 
     pp->wBitFields                    = ((s->picture_structure != PICT_FRAME) <<  0) |
-                                        (h->sps.mb_aff                        <<  1) |
+                                        ((h->sps.mb_aff &&
+                                        (s->picture_structure == PICT_FRAME)) <<  1) |
                                         (h->sps.residual_color_transform_flag <<  2) |
                                         /* sp_for_switch_flag (not implemented by Libav) */
                                         (0                                    <<  3) |