virtio: Add virtio_gpu_object_kunmap()

Implement a virtio_gpu_object_kunmap() to unmap the kernel
mapping, and use it in the TTM object destroy path.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180925161606.17980-2-ezequiel@collabora.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>