imx-drm: ipuv3-plane: enable double buffering

This allows to update the buffer base address while the DMA
channel is running. It is needed to flip the frame buffer of
an active plane.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>