drm/docs: More links for implicit/explicit fencing.

drm_atomic_set_fence_for_plane() contains the main discussion from a
driver pov, link to that from more places.

Cc: Pekka Paalanen <[e-mail]>
Reviewed-by: Pekka Paalanen <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem_framebuffer_helper.c

···
  * There is no need for &drm_plane_helper_funcs.cleanup_fb hook for simple
  * gem based framebuffer drivers which have their buffers always pinned in
  * memory.
+ *
+ * See drm_atomic_set_fence_for_plane() for a discussion of implicit and
+ * explicit fencing in atomic modeset updates.
  */
 int drm_gem_fb_prepare_fb(struct drm_plane *plane,
 			  struct drm_plane_state *state)
···
  * &dma_buf attached, extracts the exclusive fence and attaches it to plane
  * state for the atomic helper to wait on. Drivers can use this as their
  * &drm_simple_display_pipe_funcs.prepare_fb callback.
+ *
+ * See drm_atomic_set_fence_for_plane() for a discussion of implicit and
+ * explicit fencing in atomic modeset updates.
  */
 int drm_gem_fb_simple_display_pipe_prepare_fb(struct drm_simple_display_pipe *pipe,
 					      struct drm_plane_state *plane_state)

include/drm/drm_plane.h

···
 	 *
 	 * Optional fence to wait for before scanning out @fb. The core atomic
 	 * code will set this when userspace is using explicit fencing. Do not
-	 * write this directly for a driver's implicit fence, use
+	 * write this field directly for a driver's implicit fence, use
 	 * drm_atomic_set_fence_for_plane() to ensure that an explicit fence is
 	 * preserved.
 	 *