ARM: pxafb: don't disable controller on cpufreq transition if overlay in use

It's not safe to disable controller if overlay(s) is enabled (results in
system hang). So we avoid to disable controller in this case. Userspace
should choose proper governor to avoid freq changing when overlay is in
use, otherwise LCD may blink.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>