drm/radeon: Fix WARN_ON if DRM_DP_AUX_CHARDEV is enabled

Rafael Antognolli's new DRM_DP_AUX_CHARDEV feature causes a WARN_ON
if drm_dp_aux->dev == drm_connector->kdev and drm_dp_aux_unregister()
is called after drm_connector_unregister(). radeon is the only driver
affected by this besides i915. (amdgpu calls drm_dp_aux_unregister()
before drm_connector_unregister().)

Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>