drm/msm/dp: trigger unplug event in msm_dp_display_disable

1. Trigger the unplug event in msm_dp_display_disable() to shutdown audio
   properly.
2. Reset the completion before signal the disconnect event.

Fixes: 158b9aa7 ("drm/msm/dp: wait for audio notification before disabling clocks")
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Tested-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Judy Hsiao <judyhsiao@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>