drm/amd/display: Expand to higher link rates

[WHY & HOW]
To support higher link rates that sink allows, we need to make
sure driver is ready and perform correct link-training sequence.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>