drm/msm/dp: release extra mst port reference

When adding a mst port in simulator mode, an
extra reference is acquired which never gets
released. This causes the connectors to not be
destroyed during a hpd disconnect, therefore the
reference should be released.

CRs-Fixed: 2325207
Change-Id: I80e661bcefae52f9e0e57e9b234c4955b56d4d6a
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>