Use last known lux value for all automatic brightness controls

Re-introduction of lpm display states required changes to ambient
light sensor power up/down logic which then caused subtle problems
for automatic brightness filtering.

Previously the last lux value received from sensor was already cached
and used for led brightness control during sensor power off time. Now
this is done for all automatic brightness controls to avoid things
like non-filtered (=manual) display brightness setting getting
used during display unblanking.

[mce] Use last known lux value for all automatic brightness controls. Fixes NB#20910
Remove useless global variable

Also since als_lux_latest does not need to be valid, the comment
for that was wrong too.
Merge pull request #193 from spiiroin/cache_als_data

Use last known lux value for all automatic brightness controls