msm: mdss: hdmi: add audio notification acknowledge feature

Add sysfs node to allow audio HAL to acknowledge every connect/disconnect
event. This makes audio HAL and HDMI driver to be in sync with respect to
notifications and avoid any fast plug-in/out notification issues.

Change-Id: I13634bb0efb8222cdbd86afa6723781f682632b3
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>