drm/amd/display: Fix Edid emulation for linux

[Why]
EDID emulation didn't work properly for linux, as we stop programming
if nothing is connected physically.

[How]
We get a flag from DRM when we want to do edid emulation. We check if
this flag is true and nothing is connected physically, if so we only
program the front end using VIRTUAL_SIGNAL.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>