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

drivers/video/fbdev/mx3fb.c

@@ -509,7 +509,7 @@ static int sdc_init_panel(struct mx3fb_data *mx3fb, enum ipu_panel panel,
 			  uint16_t h_start_width, uint16_t h_sync_width,
 			  uint16_t h_end_width, uint16_t v_start_width,
 			  uint16_t v_sync_width, uint16_t v_end_width,
-			  struct ipu_di_signal_cfg *sig)
+			  const struct ipu_di_signal_cfg *sig)
 {
 	unsigned long lock_flags;
 	uint32_t reg;