drm/i915: Rename dp rates array as per platform

Renaming gen9_rates to skl_rates because other platforms may have different
supported rates.

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