drm/crtc: constify drm_crtc_index parameter

Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476113170-13816-1-git-send-email-jani.nikula@intel.com

include/drm/drm_crtc.h

@@ -1342,7 +1342,7 @@ extern void drm_crtc_cleanup(struct drm_crtc *crtc);
  * Given a registered CRTC, return the index of that CRTC within a DRM
  * device's list of CRTCs.
  */
-static inline unsigned int drm_crtc_index(struct drm_crtc *crtc)
+static inline unsigned int drm_crtc_index(const struct drm_crtc *crtc)
 {
 	return crtc->index;
 }