drm: Fix references to drm_plane_helper_check_state()

As of commit a01cb8ba ("drm: Move drm_plane_helper_check_state()
into drm_atomic_helper.c"), drm_plane_helper_check_state() no longer
exists, but is part of drm_atomic_helper_check_plane_state().

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/8bb42a92fc20e3d11e5847e7f15a47c687b73104.1689779916.git.geert+renesas@glider.be