drm/amd/display: Optimize fast validation cases

Optimize fast validation cases to only validate the highest voltage
level. This works because during fast validation we only care if the
mode can be supported or not (at any vlevel).

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>