u_trace: generate tracepoint index parameter in perfetto callbacks

u_trace: generate tracepoint index parameter in perfetto callbacks

Useful to figure out what's the tracepoint name you're implementing.
We'll use this in the intel perfetto integration glue to index into an
array of perfetto iid.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Felix DeGrood <felix.j.degrood@intel.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25730>