Simplify left_xy init

Originally committed as revision 21470 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.h

@@ -1144,16 +1144,14 @@ static int fill_filter_caches(H264Context *h, int mb_type){
         const int curr_mb_field_flag     = IS_INTERLACED(mb_type);
         if(s->mb_y&1){
             if (left_mb_field_flag != curr_mb_field_flag) {
-                left_xy[0] = mb_xy - s->mb_stride - 1;
-                left_xy[1] = mb_xy                - 1;
+                left_xy[0] -= s->mb_stride;
             }
         }else{
             if(curr_mb_field_flag){
                 top_xy      += s->mb_stride & (((s->current_picture.mb_type[top_xy    ]>>7)&1)-1);
             }
             if (left_mb_field_flag != curr_mb_field_flag) {
-                left_xy[0] = mb_xy                - 1;
-                left_xy[1] = mb_xy + s->mb_stride - 1;
+                left_xy[1] += s->mb_stride;
             }
         }
     }