drm/fb-helper: Allocate shadow buffer of surface height

Allocating a shadow buffer of the height of the buffer object does
not support fbdev overallocation. Use surface height instead.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-7-tzimmermann@suse.de

drivers/gpu/drm/drm_fb_helper.c

@@ -2332,7 +2332,7 @@ static int drm_fb_helper_generic_probe(struct drm_fb_helper *fb_helper,
 		return PTR_ERR(fbi);
 
 	fbi->fbops = &drm_fbdev_fb_ops;
-	fbi->screen_size = fb->height * fb->pitches[0];
+	fbi->screen_size = sizes->surface_height * fb->pitches[0];
 	fbi->fix.smem_len = fbi->screen_size;
 
 	drm_fb_helper_fill_info(fbi, fb_helper, sizes);