drm/msm/dp: reset post_open flag on cable disconnect

On physical cable disconnection, the DisplayPort session
is torn down. The flag post_open is used to manage the
state within a session. Once the session is completed,
clear this flag so that the next session is not influenced
by obsolete data.

CRs-Fixed: 2234447
Change-Id: I9ee9cde1cea30a7817ccbd42cc59c524f1ee8400
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>