x86/pmu: Pop up FW prefix to avoid out-of-context propagation

The inappropriate prefix "full-width writes" may be propagated to
later test cases if it is not popped out.

Signed-off-by: Like Xu <likexu@tencent.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-4-seanjc@google.com