drm/amd/display: Consider dp cable id only when data is non zero

Cable ID is a DP2 feature to identify max certified link rate that
a cable can carry. The cable identification method requires both
cable and display hardware support. Since the specs comes late, it is
anticipated that the first round of DP2 cables and displays may not
be fully compatible to reliably return cable ID data. Therefore the
decision of our cable id policy is that if the cable can return non
zero cable id data, we will take cable's link rate capability into
account. However if we get zero data, the cable link rate capability
is considered inconclusive. In this case, we will not take cable's
capability into account to avoid of over limiting hardware capability
from users. The max overall link rate capability is still determined
after actual dp pre-training. Cable id is considered as an auxiliary
method of determining max link bandwidth capability.

Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@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>