drm/xen-front: use simple display pipe prepare_fb helper

I missed this one because on an older tree.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: xen-devel@lists.xen.org
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409085134.27321-1-daniel.vetter@ffwll.ch