gallium/aux: Use TGSI chan name defines inplace of literals
gallium/aux: Use TGSI chan name defines inplace of literals

Signed-off-by: Edward O'Callaghan <117173807d5a49976a891944633465c0daea7f98@alterapraxis.com>
Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler