drm/ingenic: Remove dead code

The priv->ipu_plane would get a different value further down the code,
without the first assigned value being read first; so the first
assignation can be dropped.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210808134526.119198-2-paul@crapouillou.net