drm/nouveau: Add size to vbios.rom file in debugfs

With this, nvbios /sys/kernel/debug/dri/*/vbios.rom now works!

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>