drm/amd/pp: Allow underclocking when od table is empty in vbios

if max od engine clock limit and memory clock limit are not set
in vbios. driver will allow underclocking instand of disable od feature
completely.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>