daemon: Only write DDC brightness after reading and comparing first

Anecdotal evidence suggests that repeated writes to the NVRAM of some monitors will reduce its lifespan. This has recently been confirmed by vdu_controls developer Michael Hamilton: https://github.com/digitaltrails/vdu_controls/blob/master/README.md#does-adjusting-a-vdu-affect-its-lifespan-or-health

One way to reduce NVRAM writes is to avoid writing brightness if the current hardware brightness is already the same as the requested value. This commit implements such a check in DDCutilDisplay.

Some monitors may already have a similar check implemented in firmware, however, others may not. Checking first is the safe thing to do, and mirrors what recent vdu_controls releases have been doing.