drm/i915/dp: Do not grab crtc modeset lock in intel_dp_detect()

A crtc modeset lock was added for link retraining but
intel_dp_retrain_link() knows to take the necessary locks since
commit c85d200e ("drm/i915: Move SST DP link retraining into the
->post_hotplug() hook")
v2: Drop AUX power domain reference in the early return path

Fixes: c85d200e ("drm/i915: Move SST DP link retraining into the ->post_hotplug() hook")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180927205735.16651-4-dhinakaran.pandiyan@intel.com