Use const for fb_ops to let us make the fbops struct const in the
future.

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/171c573bc2977a6ec374753ac7bb03a3523ca2b7.1575390740.git.jani.nikula@intel.com