drm/i915: Report the current number of bytes freed during oom

The shrinker reports the number of pages freed, but we try to log the
number of bytes - which leads to some nonsense values being reportedly
freed during oom.

Reported-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>