drm/msm/dp: fix handling interleaved hotplug events

Fix handling of interleaved hotplug events by ensuring that
handling of a pending or current disconnect event always takes
precedent. For instance, processing of a connect event is received
while a previous disconnect event has not been processed to
completion must wait until the disconnect completes.

Change-Id: Ibb2ed1813382a53097787b4ee2d3755b06a463c0
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>