drm/amd/display: Add MST capability to trigger_hotplug interface

[Why]
Need to be able to trigger software hotplug for MST connectors

[How]
For unplug the driver calls to disable topologies manager
that connector is attached to. For plugging in it does the
whole rediscovery of all connectors in drm device and enbles their
topologies if attached.

The interface for MST connectors works in the following way:

1. To disconnect all MST topologies currently connected:
   echo 0 > /sys/kernel/debug/dri/0/amdgpu_dm_trigger_hpd_mst

2. To reconnect/rediscover all topologies that are physically
connected to the card:
   echo 1 > /sys/kernel/debug/dri/0/amdgpu_dm_trigger_hpd_mst

A related fix which has been merged with this patch
Leo Ma(Hanghong Ma)'s work:

Set power states before disable MST topology
[Why]
	When we try to disable MST topology from the
	debugfs entry, some receiver will hang.
[How]
	Set DPCD 600h power states to
	2(power down mode)before disable MST topology.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>