r600g: call cayman_emit_msaa_sample_locs only when needed
r600g: call cayman_emit_msaa_sample_locs only when needed

In the case of nr_samples <= 1, that function is (currently) a no-op anyway.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler