staging: vboxvideo: Add page-flip support

Adds crtc page-flip support by passing the new requested framebuffer
to vbox_crtc_do_set_base().

Note there is no attempt to support vblank interrupts, it's not
not known how to do this in VBOX or if it is even possible. Since
this page-flip implementation does not try to sync the page-flip
to vertical blanking, tearing effects are possible.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>