drm/virtio: Don't reinvent a flipping wheel

Now that the core helpers support nonblocking atomic commits there's
no need to invent that wheel separately (instead of fixing the bug in
the atomic implementation of virtio, as it should have been done!).

v2: Rebased on top of

commit e7cf0963
Author: Gerd Hoffmann <kraxel@redhat.com>
Date:   Tue May 31 08:50:47 2016 +0200

virtio-gpu: add atomic_commit function

Cc: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465510073-20951-1-git-send-email-daniel.vetter@ffwll.ch