drm/nouveau/nouveau: bios pointers may be unaligned, use proper accessors

This can show up on SPARC or other architectures that don't handle
unaligned accesses. The kernel normally fixes these up, but it shouldn't
have to.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96836


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