drm/i915: Remove special case from intel_supported_rates()

Now that both source and sink rates are always filled in there's no need
for any special cases in intel_supported_rates().

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>