drm/virtio: simplify virtio_gpu_alloc_cmd

Just call virtio_gpu_alloc_cmd_resp with some fixed args
instead of duplicating most of the function body.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200207074638.26386-2-kraxel@redhat.com


Reviewed-by: Chia-I Wu <olvaffe@gmail.com>