msm: mdss: update the backlight only when successful commit

During suspend and resume, the first commit got called with
no overlay buffer.   The backlight is turned on regardless
of successful commit or not.   This then causes display
corruption.

Change-Id: I00fada5f6bedabf6ab0df97f6d429297d7fb601f
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>