drm/msm/sde: fix to send idle time notification only when requested

Userspace can configure idle_time as zero when they want to
disable idle_time notifications. Currently, notification is received
on every commit till idle_time is reset to non-zero value.
Added a check to fix this.

Change-Id: Ifb4c224e2ee7bed246b975deb684a185ec702855
Signed-off-by: Sravanthi Kollukuduru <skolluku@codeaurora.org>