drm/amd/display: add support for disconnected eDP panels

[why]
On some configurations, eDP from GPU is muxed with another GPU.  DC does
not know state of mux, but DM has this knowledge.  This flag allows DC to ignore
creating EDP link when DM informs DC that EDP mux is not connected.

[how]
Add flag to dc, populate flag in DM

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