drm/radeon: fix dp link rate selection (v2)

Need to properly handle the max link rate in the dpcd.
This prevents some cases where 5.4 Ghz is selected when
it shouldn't be.

v2: simplify logic, add array bounds check

Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>