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