drm/virtio: remove virtio_gpu_alloc_object

Thin wrapper around virtio_gpu_object_create(),
but calling that directly works equally well.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-16-kraxel@redhat.com