drm/msm/dp: use drm mst helpers for mst simulator mode

Remove the redundant fxn implementation in mst simulator
module by using the drm mst helper functions for channel
and timeslot management.

CRs-Fixed: 2332059
Change-Id: I2130f0bb0e980f7a45d29b60dbadbd819d41ab44
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>