drm: rcar-du: Fix hardware plane allocation

The hardware plane allocator loops over all planes to find free
candidates. However, instead of looping over the number of hardware
planes, it loops over the number of software planes, which happens to be
larger by one unit. This has no effect in practise as the extra plane is
always cleared in the mask of free planes, but it should still be fixed
for correctness.

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