x86/pmu: Reset GP and Fixed counters during pmu_init().

In generic PMU testing, it is very common to initialize the test env by
resetting counters registers. Add helpers to reset all PMU counters for
code reusability, and reset all counters during PMU initialization for
good measure.

Signed-off-by: Like Xu <likexu@tencent.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-21-seanjc@google.com