iris: Fix Broadwell WaDividePSInvocationCountBy4

We were dividing by 4 in calculate_result_on_gpu(), and also in
iris_get_query_result().  We should stop doing the latter, and instead
divide by 4 in calculate_result_on_cpu() as well.

Otherwise, if snapshots were available, and you hit the
calculate_result_on_cpu() path, but requested it be written to a QBO,
you'd fail to get a divide.