radeonsi: extract the VGT_GS_MODE calculation into its own function
radeonsi: extract the VGT_GS_MODE calculation into its own function

Cc: "11.0 11.1" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Reviewed-by: Edward O'Callaghan <117173807d5a49976a891944633465c0daea7f98@alterapraxis.com>
Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler