drm/amd/display: dp mst detection code refactor

[why]
Move mst start top mgr in dc_link_detect layer.
Remove unused same_dpcd variable.
Move PEAK_FACTOR_X1000 and LINK_TRAINING_MAX_VERIFY_RETRY
to the proper header for defining dc link internal constant.

Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>