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: 8058944 ("drm/fbdev: Select fbdev I/O helpers from modules that require them")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20241021-fix-drm-deferred-v2-1-db1de4c6b042@linaro.org

drivers/gpu/drm/drm_fb_helper.c

@@ -697,6 +697,7 @@ void drm_fb_helper_damage_area(struct fb_info *info, u32 x, u32 y, u32 width, u3
 }
 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
@@ -740,6 +741,7 @@ void drm_fb_helper_deferred_io(struct fb_info *info, struct list_head *pagerefli
 	}
 }
 EXPORT_SYMBOL(drm_fb_helper_deferred_io);
+#endif
 
 /**
  * drm_fb_helper_set_suspend - wrapper around fb_set_suspend

include/drm/drm_fb_helper.h

@@ -256,7 +256,9 @@ void drm_fb_helper_fill_info(struct fb_info *info,
 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,
@@ -361,10 +363,12 @@ static inline int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
 	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)