drm/radeon/kms: fix legacy rmx

This makes 640x480 on my R100 work again, both
in aspect and centered mode.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Tested-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>