radeonsi: improve the computation and comment of scratch_waves
radeonsi: improve the computation and comment of scratch_waves

2% isn't much. If you think the number should be decreased, please speak up.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler