drm/i915: Hide the source vs. sink rate handling from intel_dp_compute_config()

intel_dp_compute_config() only really needs to know the rates supported
by both source and sink, so hide the raw source and sink arrays from it.

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>