drm/nouveau/bios/init: remove internal use of nvbios_init.bios

We already have a subdev pointer, from which we can locate the device's
BIOS subdev.  No need for a separate pointer.

Structure/callers not updated yet, as I want to batch more changes and
only touch the callers once.

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