i965_drv_video: fix next slice vertical position for field picture

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=38628Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>

i965_drv_video/gen7_mfd.c

@@ -718,7 +718,7 @@ gen7_mfd_avc_slice_state(VADriverContextP ctx,
         next_slice_ver_pos = first_mb_in_next_slice / width_in_mbs;
     } else {
         next_slice_hor_pos = 0;
-        next_slice_ver_pos = height_in_mbs;
+        next_slice_ver_pos = height_in_mbs / (1 + !!pic_param->pic_fields.bits.field_pic_flag);
     }
 
     BEGIN_BCS_BATCH(batch, 11); /* FIXME: is it 10??? */