KVM: Skip unnecessary "unmap" if gpc is already valid during refresh

When refreshing a gfn=>pfn cache, skip straight to unlocking if the cache
already valid instead of stuffing the "old" variables to turn the
unmapping outro into a nop.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>