freedreno: Skip CPU/GPU timestamp sync when not supported.

Fixes immediate segfaults in perfetto-enabled builds on pre-a6xx.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20660>

src/gallium/drivers/freedreno/freedreno_perfetto.cc

@@ -312,6 +312,9 @@ sync_timestamp(struct fd_context *ctx)
    uint64_t cpu_ts = perfetto::base::GetBootTimeNs().count();
    uint64_t gpu_ts;
 
+   if (!ctx->ts_to_ns)
+      return;
+
    if (cpu_ts < next_clock_sync_ns)
       return;