drm/amd/display: add support for handling 128b/132b link training test request

[why]
DP2.x added new enum values for UHBR link rates in link training test
request for test automation. We need to add UHBR link rates test request
support in preparation for compliance test automation.

[how]
added a function that translate test link rate to dc link rate.  Call
the translation function to decide the requested test link rate.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>