drm/amd/display: fix a crash on USB4 over C20 PHY

[Why]
Crash when USB4 is connected.

[How]
Added an ASIC specific code guard.

Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: Ahmad Othman <Ahmad.Othman@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>