radeonsi/gfx9: allow CMASK fast clear with RB+
radeonsi/gfx9: allow CMASK fast clear with RB+

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler