drm/amd/display: link_rate_set should index into table

[Why]
Current implementation that maps link_rate_set value to
actual link rate is incorrect.

[How]
Fix this implementation, such that link_rate_set indexes into
the supported_link_rate table.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>