drm/virtio: Remove unecessary dma_fence_ops

dma_fence_default_wait is the default now, same for the trivial
enable_signaling implementation.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180503142603.28513-15-daniel.vetter@ffwll.ch