drm/amd/display: fix dml max voltage state

Gabe's formula sometimes uses values from non-existent 'unsupported'
state to do validation.

This change adds this extra state so validation can work correctly.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>