drm/amd/display: Add support for disconnected eDP streams

drm/amd/display: Add support for disconnected eDP streams

[Why]
eDP may not be connected to the GPU on driver start causing
fail enumeration.

[How]
Move the virtual signal type check before the eDP connector
signal check.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Harry VanZyllDeJong <hvanzyll@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>