drm/i915: Assert that the exec object lookup table is a power-of-two

As we make the simplification of using a power-of-two size for the
execbuffer handle-to-object TLB, we should validate that this is actually
true and so clarify that premise.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>