drm/i915: BUG_ON put_pages later

With multiple VMs, the eviction code benefits from being able to blindly
put pages without needing to know if there are any entities still
holding on to those pages. As such it's preferable to return the -EBUSY
before the BUG.

Eviction code is the only user for now, but overall it makes sense
anyway, IMO.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>