drm/i915/gvt: Don't try to unpin an empty page range

drm/i915/gvt: Don't try to unpin an empty page range

Attempt to unpin pages in the error path of gvt_pin_guest_page() if and
only if at least one page was successfully pinned.  Unpinning doesn't
cause functional problems, but vfio_device_container_unpin_pages()
rightfully warns about being asked to unpin zero pages.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
[sean: write changelog]
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Link: https://lore.kernel.org/r/20230729013535.1070024-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>