Shenzhen Railway Station? I think time is enough. You can walk to Luohu Border from Shenzhen Station and pass the checkpoint to enter Hong Kong side. It should be nearly 6am in the morming if the train arrive on time. Then take train from Luohu to Sheung Shui. Get out of the train station and walk 100 meters to the bus terminus of Sheung Shui. There is bus A43 which will take you to Hong Kong Airport directly. It takes at most half a hour to pass Luohu Border, 10 minutes on the train to Sheung Shui and 40 minutes or so on the bus A43 to the airport.