drm/arc: Don't set the dpms hook

Doesn't do anything for atomic.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181217194303.14397-5-daniel.vetter@ffwll.ch