drm/tda998x: Don't set dpms hook

Doesn't do anything for atomic drivers.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Russell King <linux@armlinux.org.uk>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181217194303.14397-6-daniel.vetter@ffwll.ch