video: fbmem: use const pointer for fb_ops

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

v2: rebase

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/3a27f95b424a67b3542b5906c660741daf1d4ea6.1575022735.git.jani.nikula@intel.com

drivers/video/fbdev/core/fbmem.c

@@ -1079,7 +1079,7 @@ EXPORT_SYMBOL(fb_blank);
 static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
 			unsigned long arg)
 {
-	struct fb_ops *fb;
+	const struct fb_ops *fb;
 	struct fb_var_screeninfo var;
 	struct fb_fix_screeninfo fix;
 	struct fb_cmap cmap_from;
@@ -1292,7 +1292,7 @@ static long fb_compat_ioctl(struct file *file, unsigned int cmd,
 			    unsigned long arg)
 {
 	struct fb_info *info = file_fb_info(file);
-	struct fb_ops *fb;
+	const struct fb_ops *fb;
 	long ret = -ENOIOCTLCMD;
 
 	if (!info)