intel/ds: don't forget to reset upper dword timestamp read

This could lead to confusing if the 32bits roll over (every ~6mn or
so).

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 4ef6698a26 ("intel/ds: drop timestamp correlation code")
Reviewed-by: Rohan Garg <rohan.garg@intel.com>
Acked-by: Antonio Caggiano <antonio.caggiano@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13996>

src/intel/ds/intel_pps_driver.cc

@@ -167,6 +167,7 @@ void IntelDriver::enable_perfcnt(uint64_t sampling_period_ns)
 
 void IntelDriver::disable_perfcnt()
 {
+   gpu_timestamp_udw = 0;
    perf = nullptr;
    groups.clear();
    counters.clear();