msm: mdss: notify user clients when shutting down

When shutting down, notify userspace that panel state has changed.
So that it can change its state accordingly.

CRs-Fixed: 756363
Change-Id: I2b1c36742918350609b7cab83e7ad58e65db036c
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>