gpu: ipu-v3: Do not wait for DMFC FIFO to clear when disabling DMFC channel

According to basic tests, it looks there is no issue if we don't wait for
DMFC FIFO to clear when disabling DMFC channel.  NXP BSP doesn't do that,
either.  This patch is needed to avoid the annoying warning caused by a
timeout on waiting for the FIFO to clear after we add the new
DRM_PLANE_COMMIT_NO_DISABLE_AFTER_MODESET flag to the imx-drm driver
which changes the procedure to disable display channel slightly.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Peter Senna Tschudin <peter.senna@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>