st/pbo: select the right swizzle for instance IDs
st/pbo: select the right swizzle for instance IDs

The system value only has an X component, and radeonsi started
checking that in debug builds.

Reported-by: Michel Dänzer <242b79f0804cf52679d58f27ed754158c1323c75@amd.com>
Fixes: 4cf29427770f ("radeonsi: support 64-bit system values")
Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler