video: fb_defio.c: local functions should not be exposed globally

Functions not referenced outside of a source file should be marked
static to prevent them from being exposed globally.

Quiets the sparse warning:

warning: symbol 'fb_deferred_io_page' was not declared. Should it be static?
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Jaya Kumar <jayalk@intworks.biz>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>

drivers/video/fb_defio.c

@@ -23,7 +23,7 @@
 #include <linux/rmap.h>
 #include <linux/pagemap.h>
 
-struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs)
+static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs)
 {
 	void *screen_base = (void __force *) info->screen_base;
 	struct page *page;