If we use a stolen buffer, our probe callback shouldn't allocate a new
buffer; we should re-use the one from the BIOS instead if possible.

v2: fix locking (Jesse)
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>