drm/virtio: fence: pass plain pointer

Since commit "9fdd90c0 drm/virtio: add virtio_gpu_alloc_fence()"
fences are not allocated any more by virtio_gpu_fence_emit().  So there
is no need to pass down a reference to the fence pointer, a plain
pointer is enough now.

Convert virtio_gpu_fence_emit() and callers.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Robert Foss <robert.foss@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181128151021.29565-2-kraxel@redhat.com