drm/i915: Remove crtc->config references in vlv_prepare_pll

We already have a perfectly nice pipe_config, use that instead.

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/20181011100457.8776-4-maarten.lankhorst@linux.intel.com