drm/msm/dp: hold conn mode_config mutex to iterate over modes

Take the connector mode_config mutex when the DP driver iterates
over the connector's mode list in mode printing debug routine.

Change-Id: I9b8b143f8ba02a4f70c3e202ce6933a8eaf172b9
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>