video: omapfb: use const pointer for fb_ops

Use const for fb_ops to let us make the info->fbops pointer const in the
future.

Cc: linux-fbdev@vger.kernel.org
Cc: linux-omap@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/dfa4376e219ffeef9175993eaff91b5fe7ecccab.1575022735.git.jani.nikula@intel.com

drivers/video/fbdev/omap/omapfb_main.c

@@ -1052,7 +1052,7 @@ static int omapfb_ioctl(struct fb_info *fbi, unsigned int cmd,
 {
 	struct omapfb_plane_struct *plane = fbi->par;
 	struct omapfb_device	*fbdev = plane->fbdev;
-	struct fb_ops		*ops = fbi->fbops;
+	const struct fb_ops *ops = fbi->fbops;
 	union {
 		struct omapfb_update_window	update_window;
 		struct omapfb_plane_info	plane_info;