Check out this picture here:
Click!
That pic is of a YZ125, but they're all about the same. You need to take apart your steering assembly and check things out. Remove your bars, remove the large nut in the center of your triple clamp, loosen the pinch bolts on the upper clamp around the fork tubes, then tap it up and off using a rubber mallet. From there, you will see a funny-looking nut, #4 in the picture, that is more of a ring with slots cut in it. If it isn't finger loose, you can use a pair of channel-lock pliers to remove it. From there, just drop your forks and front wheel out, and clean and regrease the bearings.
If you want to risk it (I wouldn't on a bike with a few years on it), you can just tighten the ring until the play is gone, then reinstall the top clamp, bars, etc.
There's another thread already running about how tight the steering should be. I like mine to stay centered unless I bump it slightly, then I want it to flop to one side. There should be no detectable slop in the headset like you have now. Good luck.