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

Commit 917de180379d ("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: 917de180379d ("drm: rcar-du: Implement universal plane support")
Cc: stable@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
diff --git a/drivers/gpu/drm/rcar-du/rcar_du_plane.c b/drivers/gpu/drm/rcar-du/rcar_du_plane.c
index 3e30d84..d90dc428 100644
--- a/drivers/gpu/drm/rcar-du/rcar_du_plane.c
+++ b/drivers/gpu/drm/rcar-du/rcar_du_plane.c
@@ -390,7 +390,6 @@
 int rcar_du_planes_init(struct rcar_du_group *rgrp)
 {
 	struct rcar_du_device *rcdu = rgrp->dev;
-	unsigned int num_planes;
 	unsigned int crtcs;
 	unsigned int i;
 	int ret;
@@ -398,11 +397,11 @@
 	 /* Create one primary plane per CRTC in this group and seven overlay
 	  * planes.
 	  */
-	num_planes = rgrp->num_crtcs + 7;
+	rgrp->num_planes = rgrp->num_crtcs + 7;
 
 	crtcs = ((1 << rcdu->num_crtcs) - 1) & (3 << (2 * rgrp->index));
 
-	for (i = 0; i < num_planes; ++i) {
+	for (i = 0; i < rgrp->num_planes; ++i) {
 		enum drm_plane_type type = i < rgrp->num_crtcs
 					 ? DRM_PLANE_TYPE_PRIMARY
 					 : DRM_PLANE_TYPE_OVERLAY;