drm/xen: Don't set the dpms hook

Doesn't do anything for atomic.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: xen-devel@lists.xen.org
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181210100359.22507-7-daniel.vetter@ffwll.ch