anv: Add a descriptor_count to descriptor sets

This is useful for asserting in-bounds descriptor set access.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7180>

src/intel/vulkan/anv_private.h

@@ -2091,6 +2091,7 @@ struct anv_descriptor_set {
    /* Link to descriptor pool's desc_sets list . */
    struct list_head pool_link;
 
+   uint32_t descriptor_count;
    struct anv_descriptor descriptors[0];
 };