drm/i915: Remove crtc->config dereferences in intel_sanitize_crtc

We know the crtc is idle because we're at the beginning of sanitization,
so just dereference crtc->state 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/20181011100457.8776-7-maarten.lankhorst@linux.intel.com