drm/msm/dp: fix mst mode switch issue

During a mode switch operation, the connector is
detached from the mst bridge, but not reattached
via the atomic_best_encoder function. Cache the
old connector info in the mst bridge, and
reconnect it in the bridge_mode_set function.
Clear the old connector info in the
bridge_pre_enable to disallow new connections
from using the old configuration.

CRs-Fixed: 2325207
Change-Id: I8524be6843c4b0a9f0ba02a75aedbc51dd9e6377
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>