KVM: arm64: Simplify kvm_vcpu_get_mpidr_aff()

By definition, MPIDR_EL1 cannot be modified by the guest. This
means it is pointless to check whether this is loaded on the CPU.

Simplify the kvm_vcpu_get_mpidr_aff() helper to directly access
the in-memory value.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Tested-by: Joey Gouly <joey.gouly@arm.com>
Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230927090911.3355209-8-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>