drm/i915: constify mode in crtc_mode_fixup

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

commit e811f5ae


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>