nvk: Early-out impossible descriptor allocations

Also, return VK_ERROR_FRAGMENTED_POOL when appropriate.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28494>

src/nouveau/vulkan/nvk_descriptor_set.c

@@ -487,9 +487,14 @@ nvk_descriptor_pool_alloc(struct nvk_descriptor_pool *pool,
                           uint64_t *addr_out, void **map_out)
 {
    assert(size > 0);
+   assert(size % alignment == 0);
+
+   if (size > pool->heap.free_size)
+      return VK_ERROR_OUT_OF_POOL_MEMORY;
+
    uint64_t addr = util_vma_heap_alloc(&pool->heap, size, alignment);
    if (addr == 0)
-      return VK_ERROR_OUT_OF_POOL_MEMORY;
+      return VK_ERROR_FRAGMENTED_POOL;
 
    assert(addr >= pool->bo->offset);
    assert(addr + size <= pool->bo->offset + pool->bo->size);