Fixes compile error:

drm/r128_drm.h:156:23: error: array type has incomplete element type
  struct drm_clip_rect boxes[R128_NR_SAREA_CLIPRECTS];
Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>