auxdisplay: constify fb ops

Now that the fbops member of struct fb_info is const, we can start
making the ops const as well.

Cc: Miguel Ojeda Sandonis <miguel.ojeda.sandonis@gmail.com>
Cc: Robin van der Gracht <robin@protonic.nl>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Acked-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/31c18e3ce9d6962aabda4799b3051039ff591c92.1575390741.git.jani.nikula@intel.com