[Lenovo X250] The brightness keys are useless after 14.04.2 install on X250

[Gabriel Zhi Chen]

Steps:
1. Install 14.04.2 on Lenovo X250
2. Log in system and try to use the keys to adjust brighness
3. Install 14.04.1 on Lenovo X250 then upgrade system to 14.04.2
4. Log in system and try to use the keys to adjust brighness

Expected result:
The keys can adjust brightness.

Actual result:
Brightness keys are useless

Note:
Brightness can be adjusted by System settings -> Brightness & Lock -> Brightness bar.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: linux-image-3.16.0-31-generic 3.16.0-31.41~14.04.1
ProcVersionSignature: Ubuntu 3.16.0-31.41~14.04.1-generic 3.16.7-ckt5
Uname: Linux 3.16.0-31-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.7
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Mar 12 02:13:11 2015
InstallationDate: Installed on 2015-03-11 (0 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
SourcePackage: linux-lts-utopic
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.