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