drm/i915/dp: make link rate printing prettier

Turn

    [drm:intel_dp_print_rates] source rates: 162000,270000,540000,
    [drm:intel_dp_print_rates] sink rates: 162000,270000,
    [drm:intel_dp_print_rates] common rates: 162000,270000,

into

    [drm:intel_dp_print_rates] source rates: 162000, 270000, 540000
    [drm:intel_dp_print_rates] sink rates: 162000, 270000
    [drm:intel_dp_print_rates] common rates: 162000, 270000

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>