drm/arc: Align with simple pipe helpers

Simple pipe helpers only have an enable and disable hook, no more
mode_set_nofb. Call it from our enable hook to align with that
conversions.

Atomic helpers always call mode_set_nofb and enable together, so
there's no functional change here.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112084358.2771527-8-daniel.vetter@ffwll.ch