drm/amd/display: Enable mst when it's detected but yet to be initialized

drm/amd/display: Enable mst when it's detected but yet to be initialized

[Why]
drm_dp_mst_topology_queue_probe() is used under the assumption that
mst is already initialized. If we connect system with SST first
then switch to the mst branch during suspend, we will fail probing
topology by calling the wrong API since the mst manager is yet to
be initialized.

[How]
At dm_resume(), once it's detected as mst branc connected, check if
the mst is initialized already. If not, call
dm_helpers_dp_mst_start_top_mgr() instead to initialize mst

V2: Adjust the commit msg a bit

Fixes: bc068194f548 ("drm/amd/display: Don't write DP_MSTM_CTRL after LT")
Cc: Fangzhi Zuo <jerry.zuo@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>