radeonsi: only export from GS copy shader for vertex stream 0
radeonsi: only export from GS copy shader for vertex stream 0

When running the copy shader for vertex streams != 0, the SX does not need
any data from us (there is no rasterization for the higher vertex streams,
only streamout).

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler