i965: Update brw_save_primitives_written_counters for pre-Gen7.
i965: Update brw_save_primitives_written_counters for pre-Gen7.

Sandybridge and earlier only have a single counter.

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Iago Toral Quiroga <a847aabaac44184372f6b1cd1a59aa254daaeb0b@igalia.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler