drm/i915/gvt: Put the page reference obtained by KVM's gfn_to_pfn()

Put the struct page reference acquired by gfn_to_pfn(), KVM's API is that
the caller is ultimately responsible for dropping any reference.

Note, kvm_release_pfn_clean() ensures the pfn is actually a refcounted
struct page before trying to put any references.

Fixes: b901b252 ("drm/i915/gvt: Add 2M huge gtt support")
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yongwei Ma <yongwei.ma@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Link: https://lore.kernel.org/r/20230729013535.1070024-6-seanjc@google.com


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