drm/i915: constify mode in crtc_mode_fixup

Laurent Pinchart missed this when sending in is giant constify patch:

commit e811f5ae19043b2ac2c28e147a4274038e655598
Author: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Date:   Tue Jul 17 17:56:50 2012 +0200

    drm: Make the .mode_fixup() operations mode argument a const pointer

Acked-by; Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -3573,7 +3573,7 @@ void intel_encoder_destroy(struct drm_encoder *encoder)
 }
 
 static bool intel_crtc_mode_fixup(struct drm_crtc *crtc,
-				  struct drm_display_mode *mode,
+				  const struct drm_display_mode *mode,
 				  struct drm_display_mode *adjusted_mode)
 {
 	struct drm_device *dev = crtc->dev;