video: fbdev: make fbops member of struct fb_info a const pointer

Now that we no longer modify the fbops, or hold non-const pointers to
it, we can make it const. After this, we can start making the fbops
const all over the place.

Cc: linux-fbdev@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/700c6b52c39c6e7babaa921f583eac354714d9fc.1575390740.git.jani.nikula@intel.com

include/linux/fb.h

@@ -472,7 +472,7 @@ struct fb_info {
 	struct fb_deferred_io *fbdefio;
 #endif
 
-	struct fb_ops *fbops;
+	const struct fb_ops *fbops;
 	struct device *device;		/* This is the parent */
 	struct device *dev;		/* This is this fb device */
 	int class_flag;                    /* private sysfs flags */