radeonsi: move blend choice out of loop in si_blit_decompress_color
radeonsi: move blend choice out of loop in si_blit_decompress_color

It does not depend on the level or layer.

Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler