ACPI / video: seperate backlight control and event interface

The backlight control and event delivery functionality provided by ACPI
video module is mixed together and registered all during video device
enumeration time. As a result, the two functionality are also removed
together on module unload time or by the acpi_video_unregister function.
The two functionalities are actually independent and one may be useful
while the other one may be broken, so it is desirable to seperate the
two functionalities such that it is clear and easy to disable one
functionality without affecting the other one.

APIs to selectively remove backlight control interface and/or event
delivery functionality can be easily added once needed.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>