drm/nouveau/bios: rework vbios shadowing

Refactored to allow shadowing of VBIOS images longer than 64KiB, which
allows us to pass the VBIOS checksum test on certain boards.

There's also a workaround for reading the PROM VBIOS on some chipsets.

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