msm: mdss: send idle_time notification only if positive

User-land program can configure idle_time as zero when they want to
disable idle_time notifications. In the current implementation, driver
does not modify current idle timer if idle_time is zero but it may still
send the notification if the previous timer expires. Prevent this
notification if idle_time is set as zero.

Change-Id: Ib35b71a1f6c7f13f3efc28cf48a8c6bd6aaa0fa2
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>