drm/rcar-du: Fix buffer pitch alignment for R8A7790 DU

The R8A7790 DU seems to require a 128 bytes pitch alignment, even though
the documentation only mentions a 16 pixels alignement as for the
R8A7779 DU. Make this configurable through a device flag.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>