drm/amd/display: combine output signal and signal

output signal used to be a public member to be used by DM to override
the stream signal. Now since there is no longer separation between
public and private part of stream, they are combined. The overriding
was not working properly as well, which is addressed by this change

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