drm/sun4i: use simpler remove_conflicting_framebuffers(NULL)

Use remove_conflicting_framebuffers(NULL) instead of duplicating it.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/7d6d5bc4deac322b1351533c989cb3583e91be49.1535810304.git.mirq-linux@rere.qmqm.pl