drm/radeon/kms: for downclocking non-mobility check PERFORMANCE state

AtomBIOS tables on non-mobility GPU do not contain POWERSAVE/BATTERY.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>