drm/msm/dp: handle connect in separate thread

Execute the connection event in a workqueue to unblock
the usb driver's thread. Unblocking usb driver's thread
enables it to issue disconnect in case of fast plug-in/
plug-out of DP cable. Abort the connection process on
receiving the disconnect event gracefully.

Change-Id: I6c9dcaefaa8e6a9f686c9a715ecb9f854efeb809
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>