drm/rockchip: vop: clear DMA stop bit on RK3066

The RK3066 VOP sets a dma_stop bit when it's done scanning out a frame
and needs the driver to acknowledge that by clearing the bit.

Unless we clear it "between" frames, the RGB output only shows noise
instead of the picture. atomic_flush is the place for it that least
affects other code (doing it on vblank would require converting all
other usages of the reg_lock to spin_(un)lock_irq, which would affect
performance for everyone).

This seems to be a redundant synchronization mechanism that was removed
in later iterations of the VOP hardware block.

Fixes: f4a6de85 ("drm: rockchip: vop: add rk3066 vop definitions")
Cc: stable@vger.kernel.org
Signed-off-by: Val Packett <val@packett.cool>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624204054.5524-2-val@packett.cool