drm/amd/display: Update test link rate DPCD bit field to match spec

[Why]
An SCR was made to the DP2.0 spec that updated the bit field definition
for UHBR13.5 in the test link rate DPCD register.

[How]
Add new translation to match the SCR update. Keep old translation for
backwards compatibility.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Hersen Wu <hersenxs.wu@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>