drm/i915: prepare for fair lru eviction

This does two little changes:

- Add an alignment parameter for evict_something. It's not really great to
  whack a carefully sized hole into the gtt with the wrong alignment.
  Especially since the fallback path is a full evict.

- With the inactive scan stuff we need to evict more that one object, so
  move the unbind call into the helper function that scans for the object
  to be evicted, too.  And adjust its name.

No functional changes in this patch, just preparation.

Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>