drm/amd/display: make verified link cap not exceeding max link cap

[why]
There is a chance verified link cap can be greater than max link cap.
This causes software hang because we cannot power up PHY with link rate
that cannot handle.
The change is to guard verfieid link cap from becoming larger than max link cap
our PHy can support.

Reviewed-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>