drm/i915: Unconfuse DP link rate array names

To keep things clear rename the intel_dp->supported_rates[] to
intel_dp->sink_rates[], and rename the supported_rates[] name we used
elsewhere for the intersection of source and sink rates to
common_rates[].

Cc: Sonika Jindal <sonika.jindal@intel.com>
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>