fbdev/xen-fbfront: 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.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230428122452.4856-14-tzimmermann@suse.de

drivers/video/fbdev/xen-fbfront.c

@@ -429,7 +429,7 @@ static int xenfb_probe(struct xenbus_device *dev,
 	fb_info->pseudo_palette = fb_info->par;
 	fb_info->par = info;
 
-	fb_info->screen_base = info->fb;
+	fb_info->screen_buffer = info->fb;
 
 	fb_info->fbops = &xenfb_fb_ops;
 	fb_info->var.xres_virtual = fb_info->var.xres = video[KPARAM_WIDTH];