msm: mdss: update the power down sequence for DSI 12nm PHY

Update the DSI 12nm PHY power down sequence as per the latest
recommendation of h/w team to reduce the sleep current by
asserting the async reset.

Change-Id: Ia19068d692fd938d3817beca5016485f45ac8022
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>