drm/virtio: support mapping exported vram

Implement virtgpu specific map_dma_buf callback to support mapping
exported vram object dma-bufs. The dma-buf callback is used directly, as
vram objects don't have backing pages and thus can't implement the
drm_gem_object_funcs.get_sg_table callback.

Signed-off-by: David Stevens <stevensd@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20210813005441.608293-1-stevensd@chromium.org


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