drm/virtio: Remove redundant return type

drm/virtio: Remove redundant return type

virtio_gpu_fence_emit() always returns 0, since it
has no error paths.

Consequently no calls for virtio_gpu_fence_emit()
use the return value, and it can be removed.

Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190506091034.30289-1-robert.foss@collabora.com
Suggested-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>