msm: mdss: Do not set the brigtness level in idle mode

If the brightness is set in idle mode and the value is zero, display
would be off unintentionally. Skip the backlight control in idle mode
to avoid this problem.

Change-Id: I319169cd0681cae97e6689580ffc9d5468e0bd94
Signed-off-by: Devin Kim <dojip.kim@lge.com>