"msm8960-common: allow variable button light brightness""

in combination with:

https://gerrit.omnirom.org/#/c/1544/
https://gerrit.omnirom.org/#/c/1543/
https://gerrit.omnirom.org/#/c/1542/

custom button brightness is working as it should

Change-Id: I3a7aba59231cd37c3ae770b8a62e50dd9a6b63a6