drm/amd/display: Check UHBR13.5 cap when determining max link cap

drm/amd/display: Check UHBR13.5 cap when determining max link cap

[WHY]
UHBR13.5 support is optional, even if UHBR20 is supported by the device.
If source supports max UHBR13.5 while sink, cable and LTTPR support
UHBR20 but not UHBR13.5, UHBR10 should be used as the max link cap.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>