video: fbdev: s3c-fb: fix sparse warnings about using incorrect types

Use ->screen_buffer instead of ->screen_base to fix sparse warnings.

[ Please see commit 17a7b0b4

 ("fb.h: Provide alternate screen_base
  pointer") for details. ]

Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>