anv/pps: remove assert for double init

pps initializes perf counter multiple times, once from
GpuDataSource::register_data_source and once from
GpuDataSource::OnSetup. This is fine, except we should replace
failing assert with skip on second call.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38224>

src/intel/ds/intel_pps_driver.cc

@@ -73,7 +73,8 @@ bool IntelDriver::init_perfcnt()
     */
    this->clock_id = intel_pps_clock_id(drm_device.gpu_num);
 
-   assert(!perf && "Intel perf should not be initialized at this point");
+   if (perf)
+      return true;
 
    perf = std::make_unique<IntelPerf>(drm_device.fd);