drm/msm/sde: Send blank notification only when CRTC active changed

There are cases where multiple blank events are received
by driver. One such case is while dumping frames while
primary is on. So, add a check to notify blank events
only when CRTC active changed.

Change-Id: I5f7a2cb0019ecfb5a67f26434328124bd77eae9b
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>