disp: msm: sde: reset crtc power_event on crtc disable

Reset the power_event handle to NULL after unregistering
during crtc disable. This will avoid dangling pointer
being accessed later.

Change-Id: I73044835e8594b776eb593f2f0a4a1d2b5563531
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>