drm/nouveau/drm/nouveau: s/nouveau_backlight_exit/nouveau_backlight_fini/

More consistent with the rest of the codebase, no functional changes
here.

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