drm/fsl-dcu: Use drm_plane_helper_destroy()

Replace the driver's own function with drm_plane_helper_destroy(). No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220720083058.15371-8-tzimmermann@suse.de