drm/nouveau/bios: fix OF loading

Currently OF bios load fails for a few reasons:
 - checksum failure
 - bios size too small
 - no PCIR header
 - bios length not a multiple of 4

In this change, we resolve all of the above by ignoring any checksum
failures (since OF VBIOS tends not to have a checksum), and faking the
PCIR data when loading from OF.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>