This is needed to ensure ->is_unity is correct when the plane was
previously configured to output a multi-planar format with scaling
enabled, and is then being reconfigured to output a uniplanar format.

Fixes: fc04023f ("drm/vc4: Add support for YUV planes.")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180724133601.32114-1-boris.brezillon@bootlin.com