mdss: dp: check if sink device is connected before DP ON sequence

During cases where device coming out of pm suspend and disconnecting
the DP cable happens around the same time, it is possible that the
DP ON sequence is triggered from userspace before the disconnect event
is notified to the userspace. Add a check to make sure DP ON sequence
is executed only when the sink is connected.

Change-Id: I61d06e007358df75c5cedc26552422c7a8c8aa0b
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>