disp: msm: dp: init and deinit the lock in dp_mst_connector_list

The lock used by the dp debug driver to synchronize the maintenance
and usage of the list of mst connectors is not initialized. This
change adds the missing initialization and deinitialization calls.

Change-Id: I4c24600b47c0b90c256caff45edaea07d082908d
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>