fbdev: mx3fb: const pointer to ipu_di_signal_cfg

Laurent Pinchart <laurent.pinchart@ideasonboard.com> and
Jani Nikula <jani.nikula@intel.com> both
suggested to make the pointer to struct ipu_di_signal_cfg const.

Fix this.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Fixes: 3f6c93ec

 ("fbdev: mx3fb: avoid warning about psABI change")
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Enrico Weigelt <info@metux.net>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: linux-fbdev@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200408182926.GA21997@ravnborg.org