h264encode: correct negative POC after a new IDR
h264encode: correct negative POC after a new IDR

Signed-off-by: Austin Yuan <f447c34e4ebcef84a81b40a8d4dbc448a14e5570@intel.com>

zzhhui/libva,pma1/libva,pma1/libva,zzhhui/libva,gbeauchesne/libva,01org/iotg-lin-gfx-libva,gbeauchesne/libva,pma1/libva,zzhhui/libva,chenxianqin/vaapi-libva,01org/libva-cros,01org/iotg-lin-gfx-libva,01org/libva-cros,chenxianqin/vaapi-libva,chenxianqin/vaapi-libva,01org/iotg-lin-gfx-libva,gbeauchesne/libva,01org/libva-cros