disp: msm: sde: fix frame event signal for cwb

Submit a cwb frame event signal to notify the crtc that cwb
is completed. Currently cwb also uses the same frame done
event as primary. When a single cwb commit is completed, because
cwb is on a slower path there is a race condition in which the
subsequent frame done event for primary clears the refcount.
This fix isolates the events and removes this situation.

Change-Id: Ic3e18302eb8a497cbd7a00f271de2ab320576c83
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>