msm: mdss: set the idle only if userspace sets it

If userspace doesn't control the idle, driver will not control the idle.
And it avoids an unintended flash before blank

Bug: 18337866
Change-Id: If66d87f3e6d7fd0485fb5a34f623df37e12c74ff
Signed-off-by: Devin Kim <dojip.kim@lge.com>