API: h264 encode: add VA_RC_VBR_CONSTRAINED.
API: h264 encode: add VA_RC_VBR_CONSTRAINED.

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