msm8660: fix broken brightness control

  - Possibly due to a bad merge, the msm_fb_set_brightness() function
    did not actually make the brightness change.
  - This made changing the brightness in the GUI ineffective; only once
    the device was locked and then unlocked would the new value be
    set.

Reference CAF:
https://www.codeaurora.org/gitweb/quic/la/?p=kernel/msm.git;a=commit;h=378abbc399b2e723c5600e9b002d244a282c1f7f

Change-Id: I095626a290df659b96af646845ea8abc43ffb196