drm/i915: Get rid of crtc->config dereference in intel_dp_retrain_link

We're already using crtc_state here and made sure no modeset is
occurring by looking at conn_state->commit->hw_done, so there's
no need to dereference crtc->config.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004094604.2646-12-maarten.lankhorst@linux.intel.com