drm: Remove references to removed transitional helpers

The transitional helpers were removed a long time ago, but some
references stuck.  Remove them.

Fixes: 21ebe615 ("drm: Remove transitional helpers")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ad4a2f1f9fa7da083132f6c35469c77a3f9e2f0e.1689779916.git.geert+renesas@glider.be