drm/nouveau/bios: fix offset calculation for BMPv1 bioses

The only BIOS on record that needs the 14 offset has a bios major
version 2 but BMP version 1.01. Another bunch of BIOSes that need the 18
offset have BMP version 2.01 or 5.01 or higher. So instead of looking at the
bios major version, look at the BMP version. BIOSes with BMP version 0
do not contain a detectable script, so always return 0 for them.

See https://bugs.freedesktop.org/show_bug.cgi?id=68835

Reported-by: Mauro Molinari <mauromol@tiscali.it>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>