disp: msm: dp: update mst connector in debugfs data on replug

The debugfs api for mst control is a multi stage process where the
connector id is selected using the mst_con_id node, which is then
used on other commands such as edid_modes_mst for mode switch.
During mode switch control, the mode is set using the current id
for a display and then the switch is initiated through an unplug
and replug. But on an unplug the connector is destroyed and a new
id is assinged for that display on the subsequent plug. This
change detects this scenario and updates the cached connector id
with the id from the newly created connector.

Change-Id: Ib51566ac9e334d4a0b145b6ae549c59c537cb0cd
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>