drm/i915: Enforce execobject.alignment to be a power-of-two

Internal requirement for the alignment is that it must be a
power-of-two, so enforce rejection at the user interface to execbuffer
(which allows the caller to specify a stricter-than-expected alignment
criterion).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>