glsl: don't generate transform feedback candidate when not required
glsl: don't generate transform feedback candidate when not required

If we are not even looking for one don't bother generating a candidate
list.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler