drm/msm/sde: Prevent multiple unblank event trigger

In cases such as dynamic fps, resolution switch, currently
unblank event is triggered every time when new modeset is
required. Trigger the event only when active changed.

Change-Id: I6387f576a804999f1e916a1852a4c21719de9e91
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>