drm/plane: Remove drm_plane_init()

Open-code drm_plane_init() and remove the function from DRM. The
implementation of drm_plane_init() is a simple wrapper around a call
to drm_universal_plane_init(), so drivers can just use that instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Lyude Paul <lyude@redhat.com> # nouveau
Acked-by: Jyri Sarha <jyri.sarha@iki.fi>
Link: https://patchwork.freedesktop.org/patch/msgid/20220909105947.6487-2-tzimmermann@suse.de