drm: add more document for drm_crtc_from_index()

Add a bit more document for function drm_crtc_from_index() to cross
link it with drm_crtc_from_index(), and explain that the function is
useful in vblank code.

While at it, add cross link comment for drm_plane_from_index() as well.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483779131-19935-1-git-send-email-shawnguo@kernel.org

drivers/gpu/drm/drm_crtc.c

@@ -52,7 +52,10 @@
  * @idx: index of registered CRTC to find for
  *
  * Given a CRTC index, return the registered CRTC from DRM device's
- * list of CRTCs with matching index.
+ * list of CRTCs with matching index. This is the inverse of drm_crtc_index().
+ * It's useful in the vblank callbacks (like &drm_driver.enable_vblank or
+ * &drm_driver.disable_vblank), since that still deals with indices instead
+ * of pointers to &struct drm_crtc."
  */
 struct drm_crtc *drm_crtc_from_index(struct drm_device *dev, int idx)
 {

drivers/gpu/drm/drm_plane.c

@@ -254,7 +254,7 @@ EXPORT_SYMBOL(drm_plane_cleanup);
  * @idx: index of registered plane to find for
  *
  * Given a plane index, return the registered plane from DRM device's
- * list of planes with matching index.
+ * list of planes with matching index. This is the inverse of drm_plane_index().
  */
 struct drm_plane *
 drm_plane_from_index(struct drm_device *dev, int idx)