Fix CTS failures: dEQP-VK.clipping.user_defined.*fragmentshader_read
Fix CTS failures: dEQP-VK.clipping.user_defined.*fragmentshader_read

The startChannel should be within the range [0, 3].

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