fbcon: introduce for_each_registered_fb() helper

Following pattern is often used:

 for (i = 0; i < FB_MAX; i++) {
        if (registered_fb[i]) {
                ...
        }
 }

Therefore, as Andy's suggestion, for_each_registered_fb() helper can
be introduced to make the code easier to read and write by reducing
indentation level. It also saves few lines of code in each occurrence.

This patch convert all part here at the same time.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yisheng Xie <ysxie@foxmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: David Lechner <david@lechnology.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

include/linux/fb.h

@@ -650,6 +650,10 @@ extern struct fb_info *registered_fb[FB_MAX];
 extern int num_registered_fb;
 extern struct class *fb_class;
 
+#define for_each_registered_fb(i)		\
+	for (i = 0; i < FB_MAX; i++)		\
+		if (!registered_fb[i]) {} else
+
 extern int lock_fb_info(struct fb_info *info);
 
 static inline void unlock_fb_info(struct fb_info *info)