msm: mdss: Fix the intermittent idle control during stepping down backlight

If the idle control is called during stepping down backlight, just
peding the idle control and then control it when backlight is off.

Change-Id: I648f8dd0c15cbae09223200d25a89b2e2813c087
Siggned-off-by: Devin Kim <dojip.kim@lge.com>