video: fbdev: sm712fb: fix an issue about iounmap for a wrong address

the sfb->fb->screen_base is not save the value get by iounmap() when
the chip id is 0x720. so iounmap() for address sfb->fb->screen_base
is not right.

Fixes: 1461d667 ("staging: sm7xxfb: merge sm712fb with fbdev")
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200422160719.27763-1-zhengdejin5@gmail.com

drivers/video/fbdev/sm712fb.c

@@ -1429,6 +1429,8 @@ static int smtc_map_smem(struct smtcfb_info *sfb,
 static void smtc_unmap_smem(struct smtcfb_info *sfb)
 {
 	if (sfb && sfb->fb->screen_base) {
+		if (sfb->chip_id == 0x720)
+			sfb->fb->screen_base -= 0x00200000;
 		iounmap(sfb->fb->screen_base);
 		sfb->fb->screen_base = NULL;
 	}