arm64: KVM: PMU: Inject UNDEF exception on illegal register access

Both pmu_*_el0_disabled() and pmu_counter_idx_valid() perform checks
on the validity of an access, but only return a boolean indicating
if the access is valid or not.

Let's allow these functions to also inject an UNDEF exception if
the access was illegal.

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