staging: vboxvideo: Add fl_flag argument to vbox_fb_pin() helper

Allow specifying where to pin the framebuffer bo, so that this helper can
be used from the cursor code too.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>