fbdev: make FB_BACKLIGHT a tristate

BACKLIGHT_CLASS_DEVICE is already tristate, but a dependency
FB_BACKLIGHT prevents it from being built as a module.  There
doesn't seem to be any particularly good reason for this, so
switch FB_BACKLIGHT over to tristate.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Cc: Simon Horman <horms+renesas@verge.net.au>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ulf Magnusson <ulfalizer@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Hans de Goede <j.w.r.degoede@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

include/uapi/linux/fb.h

@@ -393,11 +393,9 @@ struct fb_cursor {
 	struct fb_image	image;	/* Cursor image */
 };
 
-#ifdef CONFIG_FB_BACKLIGHT
 /* Settings for the generic backlight code */
 #define FB_BACKLIGHT_LEVELS	128
 #define FB_BACKLIGHT_MAX	0xFF
-#endif
 
 
 #endif /* _UAPI_LINUX_FB_H */