disp: msm: dp: skip dp disconnect if already in progress

When dp disconnect is triggered, as part of the teardown process, the
driver sends a notification to audio driver and waits for the audio
driver to complete before proceeding. When back to back disconnects
are triggered currently there is no check if a disconnect is already
in progress. So the driver ends up reinitializing the signal
completion that the previous disconnect thread is waiting on causing
this issue.

This change adds a check at the beginning of disconnect handling and
bail out if a disconnect is already in progress.

Change-Id: I2eaee339408d9dbc120f8efc14d86fca0ebb0758
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>