drm/virtio: trace drm_fence_emit

For most drivers, drm_fence_init is followed by drm_fence_emit
immediately.  But for our driver, they are done separately.  We also
don't know the fence seqno until drm_fence_emit.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190429220825.156644-2-olvaffe@gmail.com


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