You don't... it's that simple. I know, the back of the box says "1080p" right?
This is a "misprint" and it has happened MULTIPLE times on MANY PS3 games. it's been verified by a few people, myself included with the demo version. It simply doesn't do 1080 resolution. The PS3 has a worthless upscaling chip that will only upscale games programmed at 960x1080 I believe, and only than can it upscale to 1920x1080. There is a WHOLE big forum thread here that has a list of 1080 supported/unsupported/ and "misprinted" games.