i965: Bump number of supported atomic counter buffers.

Now that we have dynamic binding tables there's no good reason anymore
to expose so few atomic counter buffers.  Increase it to 16.

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

src/mesa/drivers/dri/i965/brw_context.h

@@ -660,7 +660,7 @@ struct brw_gs_prog_data
 #define BRW_MAX_DRAW_BUFFERS 8
 
 /** Max number of atomic counter buffer objects in a shader */
-#define BRW_MAX_ABO 4
+#define BRW_MAX_ABO 16
 
 /**
  * Max number of binding table entries used for stream output.