drm/ingenic: Set DMA descriptor chain register when starting CRTC

Setting the DMA descriptor chain register in the probe function has been
fine until now, because we only ever had one descriptor per foreground.

As the driver will soon have real descriptor chains, and the DMA
descriptor chain register updates itself to point to the current
descriptor being processed, this register needs to be reset after a full
modeset to point to the first descriptor of the chain.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20211026181240.213806-5-paul@crapouillou.net


Tested-by: Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Christophe Branchereau <cbranchereau@gmail.com>