drm/drm: constify all struct drm_*_helper funcs pointers

They are not to be modified.

Generated using the semantic patch:

@@
@@
(
  const struct drm_crtc_helper_funcs *
|
- struct drm_crtc_helper_funcs *
+ const struct drm_crtc_helper_funcs *
)

@@
@@
(
  const struct drm_encoder_helper_funcs *
|
- struct drm_encoder_helper_funcs *
+ const struct drm_encoder_helper_funcs *
)

@@
@@
(
  const struct drm_connector_helper_funcs *
|
- struct drm_connector_helper_funcs *
+ const struct drm_connector_helper_funcs *
)

@@
@@
(
  const struct drm_plane_helper_funcs *
|
- struct drm_plane_helper_funcs *
+ const struct drm_plane_helper_funcs *
)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_plane_helper.c b/drivers/gpu/drm/drm_plane_helper.c
index 33807e0..40c1db9 100644
--- a/drivers/gpu/drm/drm_plane_helper.c
+++ b/drivers/gpu/drm/drm_plane_helper.c
@@ -401,9 +401,9 @@
 			    struct drm_plane_state *plane_state,
 			    struct drm_framebuffer *old_fb)
 {
-	struct drm_plane_helper_funcs *plane_funcs;
+	const struct drm_plane_helper_funcs *plane_funcs;
 	struct drm_crtc *crtc[2];
-	struct drm_crtc_helper_funcs *crtc_funcs[2];
+	const struct drm_crtc_helper_funcs *crtc_funcs[2];
 	int i, ret = 0;
 
 	plane_funcs = plane->helper_private;