drm/amd/display: refine to decide the verified link setting

[WHY]
We return false by default when link training fail at link loss.
It will cause we get a fail verified link caps directly.

[HOW]
Record the highest verified link caps. Use the recorded value as
the verified link caps if it is not successful in the last attempt
to avoid to use the lowest link setting.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Zhikai Zhai <zhikai.zhai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>