drm/i915: Reuse the aux_domain cached

intel_dp_detect() caches the aux_domain in the beginning of the
function as it is used twice, so lets also use it as the aux_domain
don't change in runtime.

v3: returning intel_dp_retrain_link() error insted of
connector_status_disconnected

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107235449.32264-1-jose.souza@intel.com