drm/amd/display: Allow Diagnostics test with eDP not connected

[Why]
Diagnostics DIO test with eDP not connected is required to run

[How]
Allow Diagnostics test with eDP not connected to skip link detection but
still execute DIO test

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>