drm/tve200: Pass NULL format_modifier to drm_simple_display_pipe_init

This Fixes build on branches where we already have format-modifier.

Reference: https://lists.freedesktop.org/archives/dri-devel/2017-August/151044.html
Fixes: 179c02fe

 ("drm/tve200: Add new driver for TVE200")
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Janet Morgan <janet.morgan@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Daniel Stone <daniels@collabora.com> (v2)
Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170825201612.23056-1-rodrigo.vivi@intel.com