drm: rcar-du: Add VSP1 support to the planes allocator

The R8A7790 DU can source frames directly from the VSP1 devices VSPD0
and VSPD1. VSPD0 feeds DU0/1 plane 0, and VSPD1 feeds either DU2 plane 0
or DU0/1 plane 1.

Allocate the correct fixed plane when sourcing frames from VSPD0 or
VSPD1, and allocate planes in reverse index order otherwise to ensure
maximum availability of planes 0 and 1.

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