Disable backlight when blanking panel.

This used to be the behaviour up to and including cm-11.0 (see e.g.
commit f7ce72ca), but in newer versions the backlight is only turned on
and never off (see commit 1e961c04). Fix it.

Change-Id: I6c3bf6abbb4e11dc010c360ecfe9f3bb3e2f3548
(cherry picked from commit 35493c4c)