drm/amdgpu/display/mst: adjust the naming of mst_port and port of aconnector

[why & how]
The term (i.e. port & mst_port) that we used to use in amdgpu is a bit
confusing. Rename them to mst_output_port & mst_root respectively.

Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>