msm: mdss: dp: add connected sysfs node

Add sysfs node to provide the status of display-port's cable
connection. Framework may query about the interface connection
status before powering it on.

Change-Id: I43d0d18b875c4516727f91c57cfb571e3ac91706
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>