WE can't vouch that this encounter between a Hongkong adviser and a leading cadre from China actually took place last week in Beijing, but our usually unreliable source swears he's got it right this time. So here goes. Hongkong adviser: ''Do you know, back in Hongkong I can go out into the streets of Kowloon and shout 'Chris Patten is an idiot' without any fear of getting arrested. You couldn't do that here in Beijing now, can you?'' Leading cadre: ''Of course, you can.'' Hongkong adviser: ''How do you mean?'' Leading cadre: ''Well, anybody can go out into the streets of Beijing and shout 'Chris Patten is an idiot' without getting arrested.'' Hongkong adviser (mutters under his breath): ''Ask a silly question . . . ''