iris: Initialize bo->index to -1 when importing buffers

A value of -1 means that the buffer has never been used in an execbuf
buffer list in any of our contexts.  While setting this isn't critical,
doing so will allow us to short-circuit some looping in the next patch.

Cc: mesa-stable
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26806>

src/gallium/drivers/iris/iris_bufmgr.c

@@ -1407,6 +1407,7 @@ iris_bo_gem_create_from_name(struct iris_bufmgr *bufmgr,
    bo->bufmgr = bufmgr;
    bo->gem_handle = open_arg.handle;
    bo->name = name;
+   bo->index = -1;
    bo->real.global_name = handle;
    bo->real.prime_fd = -1;
    bo->real.reusable = false;
@@ -1931,6 +1932,7 @@ iris_bo_import_dmabuf(struct iris_bufmgr *bufmgr, int prime_fd,
 
    bo->bufmgr = bufmgr;
    bo->name = "prime";
+   bo->index = -1;
    bo->real.reusable = false;
    bo->real.imported = true;
    /* Xe KMD expects at least 1-way coherency for imports */