nvc0: stick zero values for the compute invocation counts

Not quite perfect, but at least we don't end up with random values in
the query buffer.

Fixes KHR-GL45.pipeline_statistics_query_tests_ARB.functional_default_qo_values

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: 19.0 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 6adb9b38)