fbdev/au1200fb: Use struct fb_info.screen_buffer

Use info->screen_buffer when reading and writing framebuffers in
system memory. It's the correct pointer for this address space.

The struct fb_info has a union to store the framebuffer memory. This can
either be info->screen_base if the framebuffer is stored in I/O memory,
or info->screen_buffer if the framebuffer is stored in system memory.

As the driver operates on the latter address space, it is wrong to use
.screen_base and .screen_buffer must be used instead. This also gets
rid of casting needed due to not using the correct data type.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230428122452.4856-6-tzimmermann@suse.de

drivers/video/fbdev/au1200fb.c

@@ -1568,7 +1568,7 @@ static int au1200fb_init_fbinfo(struct au1200fb_device *fbdev)
 	fbi->fix.mmio_len = 0;
 	fbi->fix.accel = FB_ACCEL_NONE;
 
-	fbi->screen_base = (char __iomem *) fbdev->fb_mem;
+	fbi->screen_buffer = fbdev->fb_mem;
 
 	au1200fb_update_fbinfo(fbi);