staging: vboxvideo: Pass a new framebuffer to vbox_crtc_do_set_base

This modifies vbox_crtc_do_set_base() to take a new framebuffer to
be activated, instead of the existing framebuffer attached to the crtc.
This change allows the function to be given the new framebuffer from
a page-flip request.

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