Fix the error of offset calculation for encoding on BDW
Fix the error of offset calculation for encoding on BDW

Currently although the encoding can work well, the offset in the internal
object is calculated incorrectly. So fix it to avoid the potential issue.

Signed-off-by: Zhao Yakui <21bae65ecea50a19bd9d0fef489914760189fd5b@intel.com>

gbeauchesne/libva-intel-driver,01org/iotg-lin-gfx-va-driver,chenxianqin/vaapi-intel-driver,01org/iotg-lin-gfx-va-driver,uartie/vaapi-intel-driver,01org/iotg-lin-gfx-va-driver,gbeauchesne/libva-intel-driver,chenxianqin/vaapi-intel-driver,uartie/vaapi-intel-driver,01org/iotg-lin-gfx-va-driver,gbeauchesne/libva-intel-driver,gbeauchesne/libva-intel-driver,gbeauchesne/libva-intel-driver,uartie/vaapi-intel-driver,chenxianqin/vaapi-intel-driver,chenxianqin/vaapi-intel-driver,uartie/vaapi-intel-driver,chenxianqin/vaapi-intel-driver,uartie/vaapi-intel-driver,01org/iotg-lin-gfx-va-driver