drm/panthor: Fix an off-by-one in the heap context retrieval logic

The heap ID is used to index the heap context pool, and allocating
in the [1:MAX_HEAPS_PER_POOL] leads to an off-by-one. This was
originally to avoid returning a zero heap handle, but given the handle
is formed with (vm_id << 16) | heap_id, with vm_id > 0, we already can't
end up with a valid heap handle that's zero.

v4:
- s/XA_FLAGS_ALLOC1/XA_FLAGS_ALLOC/

v3:
- Allocate in the [0:MAX_HEAPS_PER_POOL-1] range

v2:
- New patch

Fixes: 9cca48fa ("drm/panthor: Add the heap logical block")
Reported-by: Eric Smith <eric.smith@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Eric Smith <eric.smith@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240502165158.1458959-5-boris.brezillon@collabora.com

drivers/gpu/drm/panthor/panthor_heap.c

@@ -323,7 +323,8 @@ int panthor_heap_create(struct panthor_heap_pool *pool,
 	if (!pool->vm) {
 		ret = -EINVAL;
 	} else {
-		ret = xa_alloc(&pool->xa, &id, heap, XA_LIMIT(1, MAX_HEAPS_PER_POOL), GFP_KERNEL);
+		ret = xa_alloc(&pool->xa, &id, heap,
+			       XA_LIMIT(0, MAX_HEAPS_PER_POOL - 1), GFP_KERNEL);
 		if (!ret) {
 			void *gpu_ctx = panthor_get_heap_ctx(pool, id);
 
@@ -543,7 +544,7 @@ panthor_heap_pool_create(struct panthor_device *ptdev, struct panthor_vm *vm)
 	pool->vm = vm;
 	pool->ptdev = ptdev;
 	init_rwsem(&pool->lock);
-	xa_init_flags(&pool->xa, XA_FLAGS_ALLOC1);
+	xa_init_flags(&pool->xa, XA_FLAGS_ALLOC);
 	kref_init(&pool->refcount);
 
 	pool->gpu_contexts = panthor_kernel_bo_create(ptdev, vm, bosize,