How can I prevent dark denim from bleeding onto my white top??

There should be a tag that tells you how to wash it. You might have to wash it by hand if it continues to bleed.

Wash them several times with dark clothes loads only. After a few washings, they will stop bleeding color.

Wash it first - the pants

Often times dark denim will come with a special tag that details how to take care of the jeans so that the denim stays nice and won't bleed onto things.
some tips I've seen on tags are:
wash jeans ALONE**
wash jeans inside out**
cold water always**
use woolite black and a couple tablespoons of vinegar
or if you really don't want the denim to fade:
don't wash the jeans at all :)

To set the color, wash the jeans a couple times before wearing them using the aforementioned techniques, but all jeans are different so it may be that you just can't wear white with this specific pair of jeans. Try it and see good luck