drm/nouveau/bios: split out shadow methods

We're about to need to be able to fetch additional chunks of data beyond
the primary bios image, which makes fetching a lot more complicated.

This splits out the verious shadowing routines to be nothing more than
very dumb "fetch this much data from this offset" routines, and leaves
the logic of what and how much to fetch in common code.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>