drm/amd/display: factor out dp detection link training and mst top detection

[why]
dc_link_detect_helper has multiple purpose:
1. Detect link and local sink.
2. Verify link capability by performing detection link training.
3. Start mst topology discovery and if succeeded fails the current interface.

This is difficult to maintain as item 2 has become more destructive to current
dc state. The change is to decouple these sequences to its own functions.
Eventually only item 2 is a destructive method and we will redesign this
sequence with a more unified policy across different encoder types.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>