drm/rcar-du: Fix buffer pitch alignment

The DU requires a 16 pixels pitch alignement. Make sure dumb buffers are
allocated with the correct pitch, and validate the pitch when creating
frame buffers.

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