samples: vfio-mdev: constify fb ops

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

v2: fix typo (Christophe de Dinechin)

Cc: Kirti Wankhede <kwankhede@nvidia.com>
Cc: kvm@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ddb10df1316ef585930cda7718643a580f4fe37b.1575390741.git.jani.nikula@intel.com