drm/amd/display: Fix MST downstream display not light up regression

The fix is for the scenario: MST+SST chain together, and hook up
to the card.

Regression: Per stream validate_context build v2
Before dc_add_stream_to_ctx(), need to update/reset
stream signal type to SIGNAL_TYPE_DISPLAY_PORT_MST. Otherwise,
downstream device cannot be added to ctx due to the lack of signal
type info (SIGNAL_TYPE_NONE).
update_stream_signal() needs to be done after stream is created.

Signed-off-by: Jerry Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>