staging: vboxvideo: Move bo_[un]resere calls into vbox_bo_[un]pin

We always need to reserve the bo around a pin / unpin, so move the
reservation there.

This allows removing the vbox_fb_[un]pin helpers.

Note this means that we now no longer hold the bo reserved while
k[un]mapping it in vbox_fb.c this is fine as it is not necessary
to hold it reserved for this.

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