Redundant pow() in 2020-CL handling

Here, RGB is converted to R'G'B':

https://github.com/haasn/libplacebo/blob/master/src/shaders/colorspace.c#L87

However, the function already has Y'B'R' from an earlier step, so only the G' needs to be computed. I am not sure this will be faster in practice, so it needs to be benchmarked.