drm/amd/display: Add UHBR135 and UHBR20 into debugfs

Add support to manually force link rate to UHBR135 (0x546) and UHBR20
(0x7d0).

Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>