drm/amd/display: Only consider pixle rate div policy for DCN32+

[Why and How]
- Only consider pixel rate div policy for DCN32+

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>