drm: rcar-du: Update framebuffer pitch and alignment limits for Gen3

The framebuffer pitch and alignment constraints reflect the limitations
of the Gen2 DU hardware. On Gen3, the DU has no memory interface and
thus doesn't impose any constraint. The limitations come instead from
the VSP that has a limit of 65535 bytes for the pitch and no alignment
constraint. Update the checks accordingly.

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