KVM: MIPS/MMU: Convert to the gfn-based MMU notifier callbacks

Move MIPS to the gfn-based MMU notifier APIs, which do the hva->gfn
lookup in common code, and whose code is nearly identical to MIPS'
lookup.

No meaningful functional change intended, though the exact order of
operations is slightly different since the memslot lookups occur before
calling into arch code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210402005658.3024832-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>