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

[ Upstream commit 092c96a8 ]

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>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>