While you could in theory use a laptop's LCD, it might end up being more hastle than it's worth. Most laptop screens have very specific power requirements making most batteries an issue for them. I've seen mods where people use ac power for the LCD panel, but I digress.
The next issue you might have is wiring the video input. Laptop display panels aren't vga like most normal displays and definitely wouldn't work with composite without a good deal of modding.
I recommend getting a cheap display with composite and low voltage requirements. In the past I've seen people use back up monitors meant for cars since they can be run on 12v (which I think is much closer to the dreamcast's requirements).
Hope this helped!