Call acpi_video_register() in intel_opregion_init() failure path

If i915 opregion is present, the acpi_video driver doesn't register
itself immediately; it defers that until the i915 opregion code is done.
But if that *fails*, the acpi_video driver was never getting registered.
And thus I have no backlight support on my Lenovo IdeaPad S10-3.

Call acpi_video_register() on the failure path, and it works again.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Matthew Garrett <mjg@redhat.com>

drivers/gpu/drm/i915/i915_opregion.c

@@ -535,6 +535,7 @@ int intel_opregion_init(struct drm_device *dev, int resume)
 err_out:
 	iounmap(opregion->header);
 	opregion->header = NULL;
+	acpi_video_register();
 	return err;
 }