h264: fix weight denom for implicit weight tables (SNB).
h264: fix weight denom for implicit weight tables (SNB).

If weighted_bipred_idc == 2, luma and chroma weight denom (in log2 base)
shall be set to the default value on Sandy Bridge. i.e. 5 as per defined
at 8-279 in AVC specs.

https://bugs.freedesktop.org/show_bug.cgi?id=40820

Signed-off-by: Gwenole Beauchesne <729713629d9640cc095f1c77c692e1881f8093c4@intel.com>

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