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 <[e-mail]>
Acked-by: Jesse Barnes <[e-mail]>
Acked-by: Matthew Garrett <[e-mail]>

drivers/gpu/drm/i915/i915_opregion.c

···
 err_out:
 	iounmap(opregion->header);
 	opregion->header = NULL;
+	acpi_video_register();
 	return err;
 }