video: fbdev: wm8505fb: 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. ]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/567cba81-5fec-4d91-f711-c0bdbfe5b513@samsung.com

drivers/video/fbdev/wm8505fb.c

@@ -339,7 +339,7 @@ static int wm8505fb_probe(struct platform_device *pdev)
 
 	fbi->fb.fix.smem_start		= fb_mem_phys;
 	fbi->fb.fix.smem_len		= fb_mem_len;
-	fbi->fb.screen_base		= fb_mem_virt;
+	fbi->fb.screen_buffer		= fb_mem_virt;
 	fbi->fb.screen_size		= fb_mem_len;
 
 	fbi->contrast = 0x10;