glsl: only set buffer block binding once during initialisation
glsl: only set buffer block binding once during initialisation

Since 8683d54d2be825 there is now a single instance of the buffer
block information that needs to be updated rather than one instance
for each stage.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler