[UPSTREAM] Move backlight handling out of pcf50633 driver

[Werner Almesberger]

Andy Green wrote:
> What do you mean "worse"?

Forced migration. Most people don't expect random breakage when
upgrading the kernel. It doesn't matter who's fault it is - the
result is a system that doesn't behave as it should, which is
experienced as a regression.

The story is different when going to a new platform. Everyone
knows that this introduces lots of changes, so the expectation
is there that certain things will need fixing.

- Werner