drm/amd/display: skip enable stream on disconnected display

[why]
Virtual signal means there is no display attached.
In this case we will assign a virtual signal type to the stream.
We should only enable the front end of the stream but not the back end.

[how]
When stream is enabling with virtual signal type, skip backend programming.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>