drm/virtio: Remove open-coded commit-tail function

Exactly matches the one in the helpers.

This avoids me having to roll out dma-fence critical section
annotations to this copy.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
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/20200709123339.547390-2-daniel.vetter@ffwll.ch