drm/ingenic: Switch IPU plane to type OVERLAY

It should have been an OVERLAY from the beginning. The documentation
stipulates that there should be an unique PRIMARY plane per CRTC.

Fixes: fc1acf31 ("drm/ingenic: Add support for the IPU")
Cc: <stable@vger.kernel.org> # 5.8+
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20210329175046.214629-2-paul@crapouillou.net