vgaarb: Wrap vga_(get|put) in CONFIG_VGA_ARB

Fix link failure without the vga arbitrator.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>