h264: set {luma,chroma}_log2_weight_denom to 0 if no pred_weight_table().
h264: set {luma,chroma}_log2_weight_denom to 0 if no pred_weight_table().

Force luma_log2_weight_denom and chroma_log2_weight_denom to zero if
there is no pred_weight_table() that was parsed.

This is a workaround for the VA intel-driver on Ivy Bridge.

ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi