[UPSTREAM] Move backlight handling out of pcf50633 driver

[Andy Green]

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Somebody in the thread at some point said:
| Andy Green wrote:
|> Balaji, if you have some interest and time, please implement this 0-255
|> in a patch and give it some small tries down /sys and see what the facts
|> are.
|
| How does that help determine the impact to existing user code?

Code that jut has the literal "64" in it for something that is variable
can take this opportunity to get fixed.  Also note we are working up at
*-tracking on this, so it is not going to be on peoples' devices
overnight randomly.

Werner's saying incrementing things will break with non 1:1 hardware
step and logical backlight level.  Is it true, or just more "plausible
bullshit"?  That is something that can be determined / fixed.

- -Andy
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