drm/nouveau/bl: Fix oops on driver unbind

Unbinding nouveau on a dual GPU MacBook Pro oopses because we iterate
over the bl_connectors list in nouveau_backlight_exit() but skipped
initializing it in nouveau_backlight_init().  Stacktrace for posterity:

    BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
    IP: nouveau_backlight_exit+0x2b/0x70 [nouveau]
    nouveau_display_destroy+0x29/0x80 [nouveau]
    nouveau_drm_unload+0x65/0xe0 [nouveau]
    drm_dev_unregister+0x3c/0xe0 [drm]
    drm_put_dev+0x2e/0x60 [drm]
    nouveau_drm_device_remove+0x47/0x70 [nouveau]
    pci_device_remove+0x36/0xb0
    device_release_driver_internal+0x157/0x220
    driver_detach+0x39/0x70
    bus_remove_driver+0x51/0xd0
    pci_unregister_driver+0x2a/0xa0
    nouveau_drm_exit+0x15/0xfb0 [nouveau]
    SyS_delete_module+0x18c/0x290
    system_call_fast_compare_end+0xc/0x6f

Fixes: b53ac1ee ("drm/nouveau/bl: Do not register interface if Apple GMUX detected")
Cc: stable@vger.kernel.org # v4.10+
Cc: Pierre Moreau <pierre.morrow@free.fr>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>