sample/vfio-mdev/mbocs: Remove dma_buf_k(un)map support

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Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kirti Wankhede <kwankhede@nvidia.com>
Cc: kvm@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-15-daniel.vetter@ffwll.ch