If we specify an unobtainable alignment (e.g, 63bits) the kernel will
eviction the object from the GTT and fail to rebind it. We can use this,
to measure how long it takes to move objects around in the GTT by
running execbuf followed by the unbind. For small objects, this will be
dominated by the nop execution time, but for larger objects this will be
ratelimited by how fast we can rewrite the PTE.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>