pmu-sw-incr in ubuntu_kvm_unit_tests failed with B-4.15 ARM64

Issue found on 4.15.0-157.164 with ARM64 node appleton-kernel

Test failed with:
 FAIL: pmu: pmu-sw-incr: PWSYNC does not increment if PMCR.E is unset

Test log:
Running '/home/ubuntu/autotest/client/tmp/ubuntu_kvm_unit_tests/src/kvm-unit-tests/tests/pmu-sw-incr'
 BUILD_HEAD=1593e88a
 timeout -k 1s --foreground 90s /usr/bin/qemu-system-aarch64 -nodefaults -machine virt,gic-version=host,accel=kvm -cpu host -device virtio-serial-device -device virtconsole,chardev=ctd -chardev testdev,id=ctd -device pci-testdev -display none -serial stdio -kernel /tmp/tmp.Yazs8muK7o -smp 1 -append pmu-sw-incr # -initrd /tmp/tmp.ORMVLcQAZh
 INFO: PMU version: 0x1
 INFO: PMU implementer/ID code: 0x41("A")/0x2
 INFO: Implements 6 event counters
 INFO: pmu: pmu-sw-incr: SW_INCR counter #0 has value 84
 FAIL: pmu: pmu-sw-incr: PWSYNC does not increment if PMCR.E is unset
 PASS: pmu: pmu-sw-incr: counter #1 after + 100 SW_INCR
 PASS: pmu: pmu-sw-incr: counter #0 after + 100 SW_INCR
 INFO: pmu: pmu-sw-incr: counter values after 100 SW_INCR #0=84 #1=100
 PASS: pmu: pmu-sw-incr: overflow on counter #0 after 100 SW_INCR
 SUMMARY: 4 tests, 1 unexpected failures
 FAIL pmu-sw-incr (4 tests, 1 unexpected failures)

This issue can be found with 4.15.0-154.161 as well. Not a regression.