drm/vboxvideo: Make structure vbox_fb_helper_funcs constant

The static structure vbox_fb_helper_funcs, of type drm_fb_helper_funcs,
is used only when it is passed as the third argument to
drm_fb_helper_fbdev_setup(), which does not modify it. Hence make it
constant to protect it from unintended modifications.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813062548.24770-1-nishkadg.linux@gmail.com