drm/ingenic: Fix non-OSD mode

Even though the JZ4740 did not have the OSD mode, it had (according to
the documentation) two DMA channels, but there is absolutely no
information about how to select the second DMA channel.

Make the ingenic-drm driver work in non-OSD mode by using the
foreground0 plane (which is bound to the DMA0 channel) as the primary
plane, instead of the foreground1 plane, which is the primary plane
when in OSD mode.

Fixes: 3c9bea4e ("drm/ingenic: Add support for OSD mode")
Cc: <stable@vger.kernel.org> # v5.8+
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210124085552.29146-5-paul@crapouillou.net