drm/amd/display: add flag to allow diag to force enumerate edp

[why]
SLT tests require that diag can drive eDP even if nothing is connected, this is not
typical production use case, so we need to add flag

[how]
add flag, and this flag supercedes "should destroy" logic

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>