drm/vc4: Fix scaling of uni-planar formats

When using uni-planar formats (like RGB), the scaling parameters are
stored in plane 0, not plane 1.

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/20180507121303.5610-1-boris.brezillon@bootlin.com