powerpc/kvm: Fix build failure with HV KVM and CBE

When running with HV KVM and CBE config options enabled, I get
build failures like the following:

  arch/powerpc/kernel/head_64.o: In function `cbe_system_error_hv':
  (.text+0x1228): undefined reference to `do_kvm_0x1202'
  arch/powerpc/kernel/head_64.o: In function `cbe_maintenance_hv':
  (.text+0x1628): undefined reference to `do_kvm_0x1602'
  arch/powerpc/kernel/head_64.o: In function `cbe_thermal_hv':
  (.text+0x1828): undefined reference to `do_kvm_0x1802'

This is because we jump to a KVM handler when HV is enabled, but we
only generate the handler with PR KVM mode.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>