msm: mdss: dp: gracefully handle cable disconnect

When the downstream DP device is physically disconnected, ensure
that all pending events are removed and the event thread is parked.
Reset all the software state so that subsequent connection events
are handled correctly.

CRs-Fixed: 2034023
Change-Id: Ie94b1da903b8e78509220e373bec4ff54026885c
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>