fbdev: Use screen_buffer in fb_sys_{read,write}()

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.

Since the fb_sys_{read,write}() functions operate on the latter address
space, it is wrong to use .screen_base and .screen_buffer must be used
instead. This also gets rid of all the casting needed due to not using
the correct data type.

v2:
	* add detailed commit message (Javier)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230428122452.4856-16-tzimmermann@suse.de

drivers/video/fbdev/core/fb_sys_fops.c

@@ -39,7 +39,7 @@ ssize_t fb_sys_read(struct fb_info *info, char __user *buf, size_t count,
 	if (count + p > total_size)
 		count = total_size - p;
 
-	src = (void __force *)(info->screen_base + p);
+	src = info->screen_buffer + p;
 
 	if (info->fbops->fb_sync)
 		info->fbops->fb_sync(info);
@@ -87,7 +87,7 @@ ssize_t fb_sys_write(struct fb_info *info, const char __user *buf,
 		count = total_size - p;
 	}
 
-	dst = (void __force *) (info->screen_base + p);
+	dst = info->screen_buffer + p;
 
 	if (info->fbops->fb_sync)
 		info->fbops->fb_sync(info);