softpipe: count CS invocations for pipeline stats queries.

Part of fixing KHR-GL33.pipeline_statistics_query_tests_ARB.
functional_compute_shader_invocations

(also requires a test bugfix: https://gitlab.khronos.org/Tracker/vk-gl-cts/-/issues/2694).

Fixes: e749c30c ("softpipe: add support for compute shaders. (v2)")