drm/panthor: Fix access to uninitialized variable in tick_ctx_cleanup()

The group variable can't be used to retrieve ptdev in our second loop,
because it points to the previously iterated list_head, not a valid
group. Get the ptdev object from the scheduler instead.

Cc: <stable@vger.kernel.org>
Fixes: d72f049 ("drm/panthor: Allow driver compilation")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Closes: https://lore.kernel.org/r/202409302306.UDikqa03-lkp@intel.com/
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930163742.87036-1-boris.brezillon@collabora.com

drivers/gpu/drm/panthor/panthor_sched.c

@@ -2046,6 +2046,7 @@ static void
 tick_ctx_cleanup(struct panthor_scheduler *sched,
 		 struct panthor_sched_tick_ctx *ctx)
 {
+	struct panthor_device *ptdev = sched->ptdev;
 	struct panthor_group *group, *tmp;
 	u32 i;
 
@@ -2054,7 +2055,7 @@ tick_ctx_cleanup(struct panthor_scheduler *sched,
 			/* If everything went fine, we should only have groups
 			 * to be terminated in the old_groups lists.
 			 */
-			drm_WARN_ON(&group->ptdev->base, !ctx->csg_upd_failed_mask &&
+			drm_WARN_ON(&ptdev->base, !ctx->csg_upd_failed_mask &&
 				    group_can_run(group));
 
 			if (!group_can_run(group)) {
@@ -2077,7 +2078,7 @@ tick_ctx_cleanup(struct panthor_scheduler *sched,
 		/* If everything went fine, the groups to schedule lists should
 		 * be empty.
 		 */
-		drm_WARN_ON(&group->ptdev->base,
+		drm_WARN_ON(&ptdev->base,
 			    !ctx->csg_upd_failed_mask && !list_empty(&ctx->groups[i]));
 
 		list_for_each_entry_safe(group, tmp, &ctx->groups[i], run_node) {