Set default value 0xF to channelWriteMask
Set default value 0xF to channelWriteMask

This change is to fix compatibility with the old dumped pipelines,
the channelWriteMask may not set in those pipelines, then the offline
compilation tools will not work properly.

googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc