arm64: KVM: PMU: Refactor pmu_*_el0_disabled

There is a lot of duplication in the pmu_*_el0_disabled helpers,
and as we're going to modify them shortly, let's move all the
common stuff in a single function.

No functional change.

Reviewed-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>