drm/fbdev: fix drm_fb_helper_deferred_io() build failure

The drm_fb_helper_deferred_io() uses struct fb_deferred_io_pageref,
which isn't available without CONFIG_FB_DEFERRED_IO. Put the function
under corresponding #ifdef to fix build failure if deferred I/O isn't
enabled.

Fixes: 8058944f5226 ("drm/fbdev: Select fbdev I/O helpers from modules that require them")
Signed-off-by: Dmitry Baryshkov <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_fb_helper.c

···
 }
 EXPORT_SYMBOL(drm_fb_helper_damage_area);
 
+#ifdef CONFIG_FB_DEFERRED_IO
 /**
  * drm_fb_helper_deferred_io() - fbdev deferred_io callback function
  * @info: fb_info struct pointer
···
 	}
 }
 EXPORT_SYMBOL(drm_fb_helper_deferred_io);
+#endif
 
 /**
  * drm_fb_helper_set_suspend - wrapper around fb_set_suspend

include/drm/drm_fb_helper.h

···
 void drm_fb_helper_damage_range(struct fb_info *info, off_t off, size_t len);
 void drm_fb_helper_damage_area(struct fb_info *info, u32 x, u32 y, u32 width, u32 height);
 
+#ifdef CONFIG_FB_DEFERRED_IO
 void drm_fb_helper_deferred_io(struct fb_info *info, struct list_head *pagereflist);
+#endif
 
 void drm_fb_helper_set_suspend(struct drm_fb_helper *fb_helper, bool suspend);
 void drm_fb_helper_set_suspend_unlocked(struct drm_fb_helper *fb_helper,
···
 	return 0;
 }
 
+#ifdef CONFIG_FB_DEFERRED_IO
 static inline void drm_fb_helper_deferred_io(struct fb_info *info,
 					     struct list_head *pagelist)
 {
 }
+#endif
 
 static inline void drm_fb_helper_set_suspend(struct drm_fb_helper *fb_helper,
 					     bool suspend)