drm: rcar-du: Clip planes to screen boundaries

Unlike the KMS API, the hardware doesn't support planes exceeding the
screen boundaries or planes being located fully off-screen. We need to
clip plane coordinates to support the use case.

Fortunately the DRM core offers a drm_atomic_helper_check_plane_state()
helper that validates the scaling factor and clips the plane
coordinates. Use it to implement the plane atomic check and use the
clipped source and destination rectangles from the plane state instead
of the unclipped source and CRTC coordinates to configure the device.

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