radeonsi: move declaring streamout parameters to its own function
radeonsi: move declaring streamout parameters to its own function

It will be reused later.

Reviewed-by: Michel Dänzer <242b79f0804cf52679d58f27ed754158c1323c75@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler