i965/fs: Use SIMD8 SSBO GET_BUFFER_SIZE message regardless of the dispatch width.
i965/fs: Use SIMD8 SSBO GET_BUFFER_SIZE message regardless of the dispatch width.

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler