MIPS: kvm: Fix build error with KVM_MIPS_DEBUG_COP0_COUNTERS enabled

Commit e4de2057 ("MIPS: KVM: Fix NULL pointer dereference") missed
converting one place accessing cop0 registers, which results in a build
error, if KVM_MIPS_DEBUG_COP0_COUNTERS is enabled.

Fixes: e4de2057 ("MIPS: KVM: Fix NULL pointer dereference")
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>