drm: rcar-du: Fix crash with groups that have less than 9 planes

Commit 917de180 ("drm: rcar-du: Implement universal plane support")
made the number of planes per group dynamic, but didn't update all loops
over the planes array, resulting in out-of-bound accesses on DU
instances that have an odd number of CRTCs (such as the R8A7790). Fix
it.

Fixes: 917de180 ("drm: rcar-du: Implement universal plane support")
Cc: stable@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>