drm/amd/display: Update DPRX detection.

[Why]
Some extra provisions are required during DPRX detection for links which
lack physical HPD and AUX/DDC pins.

[How]
Avoid attempting to access nonexistent physical pins during DPRX
detection.

Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>