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

[ Please see commit 17a7b0b4 ("fb.h: Provide alternate screen_base
  pointer") for details. ]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patchwork.freedesktop.org/patch/msgid/c687dbc5-cf5a-9508-2a61-e757a1a14568@samsung.com