drm/sysfs: Remove stale comments about calling drm_sysfs_connector_add() multiple times

drm_connector_sysfs_add() explicitly checks if connector->kdev
is already populated and returns success. So it clearly now allows
being called multiple times. Remove some stale comments to the contrary.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>