drm/radeon: only adjust default clocks on NI GPUs

commit 2e3b3b10 upstream.

SI asics store voltage information differently so we
don't have a way to deal with it properly yet.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>