drm/amd/display: support access ddc for mst branch

[Why]
Megachip dockings accesses ddc line through display driver when
installing FW. Previously, we would fail every transaction because
link attached to mst branch did not have their ddc transaction type
set.

[How]
Set ddc transaction type when mst branch is connected.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>