drm/udl: Clear struct drm_connector_funcs.dpms

Atomic modesetting doesn't use struct drm_connector_funcs.dpms and
the set function, drm_helper_connector_dpms(), wouldn't support it
anyway. So keep the pointer to NULL.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200207141602.4760-2-tzimmermann@suse.de