drm/i915: Don't copy sink rates either

Once we've read the rates from the sink we don't have to mess with them,
so the caller can just look at the stored rates without doing extra
copies. If the sink doesn't support the new link rate stuff, we just
point the caller at the default_rates[] array.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>