drm: rcar-du: Fix pitch handling for fully planar YUV formats

When creating a frame buffer, the driver verifies that the pitches for
the chroma planes match the luma plane. This is done incorrectly for
fully planar YUV formats, without taking horizontal subsampling into
account. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>