The GT 550m simply isn't fast enough to play Witcher 2 at high resolutions. Witcher 2 is among the most graphically demanding games out there, and most laptop GPUs simply can't hack it at 1080p resolution. The DDR3 on the GPU isn't helping matters, but even if your 550m were equipped with DDR5, it wouldn't make enough of a difference to let you play at 1080p.
Your options are to play the game at 720p, or get a better laptop if you really want to play at 1080p resolution. To pull off high settings at 1080p and maintain a decent framerate, you would probably need a GTX 675m or Radeon HD 7950m at minimum. Laptops that have those GPUs tend to be prohibitively expensive though if you don't have lots of cash lying around.