radv: fix crash in shader tracing.

Enabling tracing, and then having a vmfault, can leads to a segfault
before we print out the traces, as if a meta shader is executing
and we don't have the NIR for it.

Just pass the stage and give back a default.

Fixes: 9b9ccee4 ("radv: take LDS into account for compute shader occupancy stats")
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 2ac2b987)