video: fbdev: atyfb: modify the static fb_ops directly

Avoid modifying the fb_ops via info->fbops to let us make the pointer
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/7dfbf1d47203157f5eb9a6f447f0095765d0b5e6.1575390740.git.jani.nikula@intel.com